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_mech.h

@@ -701,9 +701,6 @@ gss_cred_id_t
 _gss_mg_find_mech_cred(gss_const_cred_id_t cred_handle,
 		       gss_const_OID mech_type);
 
-GSSAPI_LIB_FUNCTION gss_const_OID
-GSSAPI_CALLCONV gss_mg_name_to_oid(const char *);
-
 #include <krb5.h>
 
 /*