Bump WARNS=6 by adding const constraint and initialize the pointer

to NULL at start.

usr.bin/mkfifo/Makefile

@@ -2,5 +2,6 @@
 # $FreeBSD$
 
 PROG=	mkfifo
+WARNS?=	6
 
 .include <bsd.prog.mk>

usr.bin/mkfifo/mkfifo.c

@@ -65,8 +65,8 @@ static int f_mode;
 int
 main(int argc, char *argv[])
 {
-	char *modestr;
-	void *modep;
+	const char *modestr = NULL;
+	const void *modep;
 	mode_t fifomode;
 	int ch, exitval;