gdb: fix post-hook execution for remote targets

gdb: fix post-hook execution for remote targets

Commit b5661ff2 ("gdb: fix possible use-after-free when
executing commands") attempted to fix possible use-after-free
in case command redefines itself.

Commit 37e5833d ("gdb: fix command lookup in execute_command ()")
updated the previous fix to handle subcommands as well by using the
original command string to lookup the command again after its execution.

This fixed the test in gdb.base/define.exp but it turned out that it
does not work (at least) for "target remote" and "target extended-remote".

The problem is that the command buffer P passed to execute_command ()
gets overwritten in dont_repeat () while executing "target remote"
command itself:

	#0  dont_repeat () at top.c:822
	#1  0x000055555730982a in target_preopen (from_tty=1) at target.c:2483
	#2  0x000055555711e911 in remote_target::open_1 (name=0x55555881c7fe ":1234", from_tty=1, extended_p=0)
	    at remote.c:5946
	#3  0x000055555711d577 in remote_target::open (name=0x55555881c7fe ":1234", from_tty=1) at remote.c:5272
	#4  0x00005555573062f2 in open_target (args=0x55555881c7fe ":1234", from_tty=1, command=0x5555589d0490)
	    at target.c:853
	#5  0x0000555556ad22fa in cmd_func (cmd=0x5555589d0490, args=0x55555881c7fe ":1234", from_tty=1)
	    at cli/cli-decode.c:2737
	#6  0x00005555573487fd in execute_command (p=0x55555881c802 "4", from_tty=1) at top.c:688

Therefore the second call to lookup_cmd () at line 697 fails to find
command because the original command string is gone.

This commit addresses this particular problem by creating a *copy* of
original command string for the sole purpose of using it after command
execution to lookup the command again. It may not be the most efficient
way but it's safer given that command buffer is shared and overwritten
in hard-to-foresee situations.

Tested on x86_64-linux.

PR 30249
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30249

Approved-By: Tom Tromey <tom@tromey.com>