LibC: Implement flock(2) using fcntl's F_SETLK

While flock is not a posix interface, it exists on linux and all BSDs as
far as I am aware.

Userland/Libraries/LibC/sys/file.cpp

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2021, Peter Elliott <pelliott@ualberta.ca>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Assertions.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <sys/file.h>
+
+extern "C" {
+
+int flock(int fd, int operation)
+{
+    struct flock lock {
+        short(operation & 0b11), SEEK_SET, 0, 0, 0
+    };
+    if (operation & LOCK_NB) {
+        return fcntl(fd, F_SETLK, &lock);
+    }
+
+    // FIXME: Implement F_SETLKW in fcntl.
+    VERIFY_NOT_REACHED();
+}
+}