Userland: Add a dummy passthrough "flock" program

This allows you to run our build system's Makefiles inside SerenityOS
itself (since they rely on "flock")

Obviously it doesn't do any locking as we don't support that yet.

Userland/flock.cpp

@@ -0,0 +1,26 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+int main(int argc, char** argv)
+{
+    if (argc < 3) {
+        printf("usage: flock <path> <command...>\n");
+        return 0;
+    }
+
+    if (!fork()) {
+        if (execvp(argv[2], &argv[2]) < 0) {
+            perror("execvp");
+            exit(1);
+        }
+    }
+
+    int status;
+    if (waitpid(-1, &status, 0) < 0) {
+        perror("waitpid");
+        return 1;
+    }
+    return WEXITSTATUS(status);
+}