gss: merge gss_name_to_oid and gss_mg_name_to_oid

The recently introduced gss_mg_name_to_oid() function supported looking up
dynamically loaded mechanisms by name, but did not support partial matches or
the legacy "Kerberos 5" name as supported by gss_name_to_oid().

Consolidate these into a single function, and also add support for dynamically
loaded mechanisms to gss_oid_to_name().

API behavior difference: the Kerberos mechanism is now referred to by "krb5"
rather tha "Kerberos 5", although for legacy compatibility gss_name_to_oid()
will recognize the old name. However, gss_oid_to_name() will return "krb5". The
anticipated impact is minimal as these are not standard GSS-APIs and do not
appear to have any public usage outside Heimdal.

lib/gssapi/gssapi/gssapi.h

@@ -1144,9 +1144,12 @@ GSSAPI_LIB_FUNCTION OM_uint32 GSSAPI_LIB_CALL gss_duplicate_cred (
             gss_const_cred_id_t /*input_cred_handle*/,
             gss_cred_id_t * /*output_cred_handle*/
            );
+
+/* Return a mechanism short name from an OID */
 GSSAPI_LIB_FUNCTION const char * GSSAPI_LIB_CALL
 gss_oid_to_name(gss_const_OID oid);
 
+/* Return a mechanism OID from a short name or dotted OID */
 GSSAPI_LIB_FUNCTION gss_OID GSSAPI_LIB_CALL
 gss_name_to_oid(const char *name);