xargs: disallow -R0 and -L0

Both cases were interpreted as these flags are unset. This meant that
-R0 got converted to -R5 and that -L0 didn't have any effect at all.
Since make at most 0 replacements isn't useful and since call utility
for every 0 lines read doesn't make sense, throw an error for these
two cases.

MFC after:	1 week
Reviewed by:	des, kevans
Differential Revision:	https://reviews.freebsd.org/D41022

usr.bin/xargs/xargs.c

@@ -174,7 +174,7 @@ main(int argc, char *argv[])
 			replstr = optarg;
 			break;
 		case 'L':
-			Lflag = (int)strtonum(optarg, 0, INT_MAX, &errstr);
+			Lflag = (int)strtonum(optarg, 1, INT_MAX, &errstr);
 			if (errstr)
 				errx(1, "-%c %s: %s", ch, optarg, errstr);
 			break;
@@ -203,6 +203,8 @@ main(int argc, char *argv[])
 			Rflag = (int)strtonum(optarg, INT_MIN, INT_MAX, &errstr);
 			if (errstr)
 				errx(1, "-%c %s: %s", ch, optarg, errstr);
+			if (!Rflag)
+				errx(1, "-%c %s: %s", ch, optarg, "must be non-zero");
 			break;
 		case 'r':
 			/* GNU compatibility */