nextboot: check unlink, but only warn on !ENOENT

Emulate rm -f from the nextboot.sh script: Report all errors, except
ENOENT. This problems show through, except the expected one when
nextboot.conf isn't there.

Sponsored by:		Netflix
Reviewed by:		rew
Differential Revision:	https://reviews.freebsd.org/D44013

sbin/reboot/reboot.c

@@ -288,7 +288,8 @@ main(int argc, char *argv[])
 		errx(1, "-r and -k cannot be used together, there is no next kernel");
 
 	if (Dflag) {
-		(void)unlink(PATH_NEXTBOOT);	/* Say nothing if it's not there */
+		if (unlink(PATH_NEXTBOOT) != 0 && errno != ENOENT)
+			warn("unlink " PATH_NEXTBOOT);
 		exit(0);
 	}