dirmngr: LDAP search by a mailbox now ignores revoked keys.

* dirmngr/ks-engine-ldap.c (keyspec_to_ldap_filter): Ignore revoked
and disable keys in mail mode.

The LDAP schema has a revoked and a disabled flag. The former will be
set if a revoked key is uploaded; the latter can be set by other
means. With this change a search by mailbox does not anymore return
keys with these LDAP attributes set. This allows to better maintain a
directory with multiple keys per mailbox.

Doing the same for expired keys could also be done but requires more
effort.

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