Add -r option for opening the file to lock read-only

It makes a difference on *nix.

src/xpdev/lockfile.c

 #include <stdlib.h>
+#include <string.h>
 #include "filewrap.h"
 
 int main(int argc, char** argv)
 {
 	int access = O_RDWR;
-	const char* path = argv[1];
 
 	if(argc < 4) {
-		fprintf(stderr, "usage: lockfile path offset length\n");
+		fprintf(stderr, "usage: lockfile [-r] path offset length\n");
+		printf("-r     open file read-only instead of read/write\n");
 		return 0;
 	}
 
+	int argn = 1;
+	if(strcmp(argv[argn], "-r") == 0) {
+		access = O_RDONLY;
+		++argn;
+	}
+	const char* path = argv[argn++];
 	int file = open(path, access);
 	if(file < 0) {
 		puts("Open failure");
@@ -18,7 +25,7 @@ int main(int argc, char** argv)
 		return -1;
 	}
 
-	int result = lock(file, atoi(argv[2]), atoi(argv[3]));
+	int result = lock(file, atoi(argv[argn]), atoi(argv[argn + 1]));
 	if(result != 0) {
 		puts("Lock failure");
 		perror(path);