gdb: include help aliases in help command completion

gdb: include help aliases in help command completion

There are a bunch of aliases that get used with help, but the current
command completion logic does not include those when doing completions.

Since the framework is already mostly in place, extend complete_on_cmdlist
slightly to pass down the ignore_help_classes flag like is done with the
existing lookup command logic.

Now you can do:
	(gdb) help use<tab>
and get back:
	(gdb) help user-defined

Signed-off-by: Mike Frysinger <vapier@gentoo.org>