gdb: add completion handler for "handle" and "signal"

gdb: add completion handler for "handle" and "signal"

The command line completion has spoiled me.  Thus the lack of completion with
the "handle" command annoys me.  Patch!

This does a few things:
 - adds a VEC_merge helper
 - adds a generic signal completer
 - adds a completion handler for the "handle" command
 - sets the completion handler for the "signal" command

URL: http://sourceware.org/bugzilla/show_bug.cgi?id=10436
Signed-off-by: Mike Frysinger <vapier@gentoo.org>