gpg: Implement the LDAP AKL method.

* g10/keyserver.c (keyserver_import_mbox): Add arg flags and change
callers.
(keyserver_import_ldap): Remove.  It has always returned a not
implemented error since 2.1.
* g10/getkey.c (get_pubkey_byname): Repurpose LDAP to do basically the
same as KEYSERVER.

The old LDAP mechanism to locate a server via SRV records has long
been gone (since 2014) due to the dropping of the keyserver helpers.
The new purpose better reflects reality and can be used in
environments where keys are provided by an in-house LDAP server.

(cherry picked from commit 068ebb6f1eee37d31f5ffb44b7f8069d9ca3f7b8)