dirmngr: Rewrite a weird function by straighter code.

* dirmngr/ldap-parse-uri.c (ldap_uri_p): Use ascii-memcasecmp.

Note that the first test on ldaps or ldaps in the original code did
not worked at all so that the Mixed Case part took over there.

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