dirmngr: Silence log output from dirmngr_ldap.

* dirmngr/dirmngr_ldap.c: Remove assert.h.
(main): Replace assert by log_assert.
* dirmngr/ldap.c (run_ldap_wrapper): Use debug options to pass
verbose options to dirmngr_ldap.
(start_cert_fetch_ldap): Ditto.

verbose is a pretty common option in dirmngr.conf and it would clutter
the logs with output from dirmngr_ldap. Now we require DBG_EXTPROG
or DBG_LOOKUP to make dirmngr_ldap more verbose.

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