gpg: Do not use self-sigs-only for LDAP keyserver imports.

* dirmngr/ks-engine-ldap.c (ks_ldap_get): Print a SOURCE status.
* g10/options.h (opts): New field expl_import_self_sigs_only.
* g10/import.c (parse_import_options): Set it.
* g10/keyserver.c (keyserver_get_chunk): Add special options for LDAP.

I can be assumed that configured LDAP servers are somehow curated and
not affected by rogue key signatures as the HKP servers are. Thus we
can allow the import of key signature from LDAP keyservers by default.

• GnuPG-bug-id: T5387