gpg: Allow --locate-external-key even with --no-auto-key-locate.

* g10/getkey.c (akl_empty_or_only_local): New.
* g10/gpg.c (DEFAULT_AKL_LIST): New.
(main): Use it here.
(main) <aLocateExtKeys>: Set default AKL if none is set.

This better matches the expectations of the user. The used list in
this case is the default list ("local,wkd") with local ignored by the
command anyway.

• GnuPG-bug-id: T4662
• Signed-off-by: Werner Koch <wk@gnupg.org>