Add support for new dirmngr option to configure X.509 servers

This change adds support for the new "ldapserver" option of dirmngr.
The now deprecated "keyserver" option of gpgsm is still read, but
changes are always written to the new option (and the old option is
cleared). This change removes support for the ancient "LDAP Server"
pseudo-option which new versions of gpgconf no longer support.

• GnuPG-bug-id: T5801