Introduce the "with" command

  ( See original discussion and prototype here:
   https://sourceware.org/ml/gdb-patches/2019-05/msg00570.html )

 (gdb) help with
 Temporarily set SETTING to VALUE, run COMMAND, and restore SETTING.
 Usage: with SETTING [VALUE] [-- COMMAND]
 Usage: w SETTING [VALUE] [-- COMMAND]
 SETTING is any setting settable with the "set" command.
 E.g.:
   with language pascal -- print obj
   with print elements unlimited -- print obj

As can be seen above, the "with" command is just like "set", but
instead of setting the setting permanently, it sets the setting, runs
a command and then restores the setting.

 (gdb) p g_s
 $1 = {a = 1, b = 2, c = 3}
 (gdb) with language ada -- print g_s
 $2 = (a => 1, b => 2, c => 3)
 Warning: the current language does not match this frame.
 (gdb) show language
 The current source language is "auto; currently c".
 (gdb) with print elements 100 -- with print object on -- print 1
 $3 = 1

You can shorten things a bit though, as long as unambiguous.  So this:

 (gdb) with print elements 100 -- with print object off -- print 1

is the same as:

 (gdb) w p el 100 -- w p o 0 -- p 1

Note that the patch adds a "w" alias for "with", as "w" is not
currently taken:

 (gdb) w
 Ambiguous command "w": watch, wh, whatis, where, while, while-stepping, winheight, ws.

Let me know if you'd prefer to reserve "w" for one of the other
commands above.  IMHO, this command will end up being used frequently
enough that it deserves the "w" shorthand.

A nice feature is that this is fully integrated with TAB-completion:

 (gdb) with p[TAB]
 pagination  print       prompt      python
 (gdb) with print [TAB]
 address                max-depth              static-members
 array                  max-symbolic-offset    symbol
 array-indexes          null-stop              symbol-filename
 asm-demangle           object                 symbol-loading
 demangle               pascal_static-members  thread-events
 elements               pretty                 type
 entry-values           raw                    union
 frame-arguments        repeats                vtbl
 inferior-events        sevenbit-strings
 (gdb) with print [TAB]

 (gdb) with print elements unlimited -- thread apply all -
 -ascending  -c          -q          -s

 (gdb) with print elements unlimited -- print -[TAB]
 -address         -max-depth       -repeats         -vtbl
 -array           -null-stop       -static-members
 -array-indexes   -object          -symbol
 -elements        -pretty          -union

The main advantage of this new command compared to command options,
like the new "print -OPT", is that this command works with any
setting, and, it works nicely when you want to override a setting
while running a user-defined command, like:

 (gdb) with print pretty -- usercmd

The disadvantage is that it isn't as compact or easy to type.  I think
of command options and this command as complementary.  I think that
even with this new command, it makes sense to continue developing the
command options in the direction of exposing most-oft-used settings as
command options.

Inspired by Philippe's "/" command proposal, if no command is
specified, then the last command is re-invoked, under the overridden
setting:

 (gdb) p g_s
 $1 = {a = 1, b = 2, c = 3}
 (gdb) with language ada
 $2 = (a => 1, b => 2, c => 3)
 Warning: the current language does not match this frame.

Note: "with" requires "--" to separate the setting from the command.
It might be possible to do without that, but, I haven't tried it yet,
and I think that this can go in without it.  We can always downgrade
to making "--" optional if we manage to make it work.

On to the patch itself, the implementation of the command is simpler
than one might expect.  I factored out a bit from pipe_command into
repeat_previous directly, because otherwise I'd need to copy&paste the
same code and same error message in the with command.  The
parse_cli_var_uinteger/parse_cli_var_zuinteger_unlimited changes are
necessary since we can now pass an empty string as argument.

The documentation bits are originally based on Philippe's docs for the
"/" command, hence the attribution in the ChangeLog.

gdb/ChangeLog:
2019-06-14  Pedro Alves  <palves@redhat.com>

	* NEWS (New commands): Mention "with".
	* cli/cli-cmds.c (pipe_command): Adjust to new repeat_previous
	interface.
	* cli/cli-setshow.c (parse_cli_var_uinteger)
	(parse_cli_var_zuinteger_unlimited): Handle empty argument
	strings.
	* command.h (repeat_previous): Now returns const char *.  Adjust
	comment.
	* printcmd.c: Include "top.h", "cli/cli-decode.h", and
	"cli/cli-setshow.h".
	(with_command, with_command_completer): New.
	(_initialize_printcmd): Install the "with" command and its "w"
	alias.
	* top.c (repeat_previous): Move bits from pipe_command here:
	Return the saved command line, if any; error out if there's no
	command to relaunch.

gdb/doc/ChangeLog:
2019-06-14  Pedro Alves  <palves@redhat.com>
	    Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.texinfo (Command Settings): New node documenting the general
	concept of settings, how to change them, and the new "with"
	command.

gdb/testsuite/ChangeLog:
2019-06-14  Pedro Alves  <palves@redhat.com>

	* gdb.base/with.c: New file.
	* gdb.base/with.exp: New file.
---
 gdb/doc/gdb.texinfo             |  87 +++++++++++++++++++++
 gdb/NEWS                        |  12 +++
 gdb/cli/cli-cmds.c              |   7 +-
 gdb/cli/cli-setshow.c           |   4 +-
 gdb/command.h                   |  19 +++--
 gdb/printcmd.c                  | 132 +++++++++++++++++++++++++++++++
 gdb/testsuite/gdb.base/with.c   |  41 ++++++++++
 gdb/testsuite/gdb.base/with.exp | 168 ++++++++++++++++++++++++++++++++++++++++
 gdb/top.c                       |   7 +-
 9 files changed, 460 insertions(+), 17 deletions(-)
 create mode 100644 gdb/testsuite/gdb.base/with.c
 create mode 100644 gdb/testsuite/gdb.base/with.exp

Introduce the "with" command

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