[PR gdb/16238] Add completer for the show user command

The 'show user' command (which shows the definition of non-python/scheme
user defined commands) is currently missing a completer. This is
mentioned in PR 16238.  Having one can improve the user experience.

In this commit I propose an implementation for such completer as well as
the associated tests.

Tested on x86_64 GNU/Linux.

All feedbacks are welcome.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16238