gpg: expand GPG groups when resolving a key

* g10/expand-group.c: New
* g10/pkclist.c: Extract expand_group and expand_id into expand-group.c.
* g10/keydb.h: Add prototypes of expand_id and expand_group.
* g10/getkey.c: Use expand_group before resolving key references.
* g10/Makefile.am: Compile expand-group.c.

When searching a key by its name, try to expand the provided name in
case it is a GPG group reference. This GPG group resolution is performed
before the individual keys are verified.

This allows key listing using a GPG group reference. In particular, this
modification fixes the encryption to group support in KDE's Kmail which
is broken since version 18.04.

• Changed new filename to use a dash instead of an underscore.
• Indendation changes.

• Signed-off-by: Werner Koch <wk@gnupg.org>