Fix warnings and set WARNS to 6.

MFC after:	1 week

tools/tools/cfi/Makefile

@@ -3,5 +3,6 @@
 PROG=	cfi
 BINDIR=	/usr/local/bin
 NO_MAN=
+WARNS?=6
 
 .include <bsd.prog.mk>

tools/tools/cfi/cfi.c

@@ -38,9 +38,12 @@
 #include <sys/ioctl.h>
 #include <sys/cfictl.h>
 
-#include <stdio.h>
+#include <err.h>
 #include <getopt.h>
+#include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
 
 const char *progname;
 const char *dvname;
@@ -48,8 +51,6 @@ const char *dvname;
 static void
 usage(void)
 {
-	int i;
-
 	fprintf(stderr, "usage: %s [-f device] op...\n", progname);
 	fprintf(stderr, "where op's are:\n");
 	fprintf(stderr, "fact\t\tread factory PR segment\n");