gdb: add "maint set/show selftest verbose" commands and use process_options

gdb: add "maint set/show selftest verbose" commands and use process_options

I saw the new -verbose switch to "maint selftests" and thought it would
be nice for it to use the option framework.  For example, that makes
having completion easy.  It's not that high value, given this is a
maintenance command, but I had never used the framework myself, so it
was a good way to practice.

This patch also adds the "maint set/show selftest verbose" setting.  It
would be possible to use option framework without adding the setting,
but using the framework makes adding the option almost trivial, so I
thought why not.

Change-Id: I6687faa0713ff3da60b398253211777100094144