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/libgssapi-exports.def

@@ -69,7 +69,6 @@ EXPORTS
 	gss_krb5_set_allowable_enctypes
 	gss_localname
 	gss_mg_collect_error
-	gss_mg_name_to_oid
         gss_mo_get
         gss_mo_set
         gss_mo_list