dirmngr: Support new gpgNtds parameter in LDAP keyserver URLs.

* dirmngr/ldap-parse-uri.c (ldap_parse_uri): Support a new gpgNtds
extension.
* dirmngr/ks-engine-ldap.c (my_ldap_connect): Do ldap_init always with
hostname - which is NULL and thus the same if not given.  Fix minor
error in error code handling.

Note that "gpgNtds" is per RFC-4512 case insensitive and has not yet
been officially regisetered. Thus for correctness the OID can be
used:

1.3.6.1.4.1.11591.2.5          LDAP URL extensions
1.3.6.1.4.1.11591.2.5.1          gpgNtds=1 (auth. with current user)

Note that the value must be 1; all other values won't enable AD
authentication and are resevered for future use.

This has been cherry-picked from the 2.2 branch,
commit 55f46b33df08e8e0ea520ade5f73b321bc01d705

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