gss: order SPNEGO proposed mechs by req_flags

Sort the list of mechanisms proposed by the initiator so that mechanisms are
preferred by their advertised support for GSS flags. For example, if
GSS_C_MUTUAL_FLAG is requested, a mechanism that offers GSS_C_MA_AUTH_TARG will
be preferred over one that doesn't. The flag/mechanism attribute combinations
are also assigned a weight (mutual trumps anonymous, for example).

lib/gssapi/spnego/init_sec_context.c

@@ -285,6 +285,7 @@ spnego_initial(OM_uint32 * minor_status,
 
     sub = _gss_spnego_indicate_mechtypelist(&minor,
 					    ctx->target_name,
+					    req_flags,
 					    initiator_approved,
 					    &sel,
 					    0,