fetch: make -S argument accept values > 2GB

Use strtoll() to parse the argument of the -S option.

FreeBSD has supported 64 bit file offsets for more than 25 years on
all architectures and off_t is a 64 bit integer type for that reason.

While strtol() returns a 64 bit value on 64 LP64 architectures, it
is limit to 32 bit on e.g. i386. The strtoll() function returns a 64
but result on all supported architectures and therefore supports the
possible file lengths and file offsets on 32 bit archtectures.

Reported by:	antoine
MFC after:	3 days

usr.bin/fetch/fetch.c

@@ -1005,7 +1005,7 @@ main(int argc, char *argv[])
 			r_flag = 1;
 			break;
 		case 'S':
-			S_size = (off_t)strtol(optarg, &end, 10);
+			S_size = strtoll(optarg, &end, 10);
 			if (*optarg == '\0' || *end != '\0')
 				errx(1, "invalid size (%s)", optarg);
 			break;