mkfifo: Set initial mode

From POSIX:

In the symbolic_mode strings, the op characters '+' and '-'
shall be interpreted relative to an assumed initial mode of
a=rw.

mkfifo.c

@@ -14,7 +14,7 @@ usage(void)
 int
 main(int argc, char *argv[])
 {
-	mode_t mode = 0, mask;
+	mode_t mode = 0666, mask;
 	int mflag = 0, ret = 0;
 
 	ARGBEGIN {