[RFA 7/8] Use unique_xmalloc_ptr in execute_gdb_command

[Tom Tromey]

Tom> Here's the new version of this patch.

Pedro> OK.

Thanks for these reviews.  I think now all of my various c++-ification
series are fully approved, so I plan to rebase, re-test (on the
buildbot), and then push.  Once this happens (it may be a day or two)
please let me know if there are any difficulties; I'll fix them as soon
as I'm able.

Tom