gpg: Allow fingerprint based lookup with --locate-external-key.

* g10/keyserver.c (keyserver_import_fprint_ntds): New.
* g10/getkey.c (get_pubkey_byname): Detect an attempt to search by
fingerprint in no_local mode.

See the man page. For testing use

gpg --auto-key-locate local,wkd,keyserver --locate-external-key  \
  FINGERPRINT

with at least one LDAP keyserver given in dirmngr.conf. On Windows
"ntds" may be used instead or in addtion to "keyserver".

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