Make "info threads" use the gdb::option framework

Make "info threads" use the gdb::option framework

I had this in my local "print -OPT" branch, but had decided not to
include in the original series, because it's not as useful as the
others.  I'm sending it now, since I had it written anyway.

This makes "info threads" use the gdb::option framework to process
options.  There's only one option today (-gid), and it isn't used much
frequently unless you're looking at matching MI output.  Still, this
was in the neighborhood of "thread apply" so I had converted it.

The main advantage is that TAB completion now shows you the available
options, and gives you a hint to what the command accepts as operand
argument, including showing a metasyntactic variable:

  (gdb) info threads [TAB]
  -gid  ID

  (gdb) help info threads
  Display currently known threads.
  Usage: info threads [OPTION]... [ID]...

  Options:
    -gid
      Show global thread IDs.

  If ID is given, it is a space-separated list of IDs of threads to display.
  Otherwise, all threads are displayed.
  (gdb)

gdb/ChangeLog:
2019-06-13  Pedro Alves  <palves@redhat.com>

	* NEWS (Completion improvements): Mention "info threads".
	* thread.c (struct info_threads_opts, info_threads_option_defs)
	(make_info_threads_options_def_group): New.
	(info_threads_command): Use gdb::option::process_options.
	(info_threads_command_completer): New.
	(_initialize_thread): Use gdb::option::build_help to build the
	help text for "info threads".

gdb/testsuite/ChangeLog:
2019-06-13  Pedro Alves  <palves@redhat.com>

	* gdb.base/options.exp (test-info-threads): New procedure.
	(top level): Call it.