upas/marshal: use login instead of user (thanks sirjofri)

User is the upasname, and is unlikely to exist when we save
the message to the outbox. We should use the login name instead.

sys/src/cmd/upas/marshal/marshal.c

@@ -1088,7 +1088,7 @@ sendmail(Addr *to, Addr *cc, Addr *bcc, int *pid, char *rcvr)
 			case 0:
 				close(pfd[0]);
 				/* BOTCH; "From " time gets changed */
-				f = foldername(nil, user, rcvr);
+				f = foldername(nil, login, rcvr);
 				b = openfolder(f, time(0));
 				if(b != nil){
 					fd = Bfildes(b);