LibCore: Add LockFile, a filesystem based mutex

This API wraps flock(2) and also handles the file creation and deletion
when the LockFile goes out of scope.

Userland/Libraries/LibCore/LockFile.h

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2021, Peter Elliott <pelliott@ualberta.ca>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace Core {
+
+class LockFile {
+public:
+    enum class Type {
+        Exclusive,
+        Shared
+    };
+
+    LockFile(LockFile const& other) = delete;
+    LockFile(char const* filename, Type type = Type::Exclusive);
+    ~LockFile();
+
+    bool is_held() const;
+    int error_code() const { return m_errno; }
+    void release();
+
+private:
+    int m_fd { -1 };
+    int m_errno { 0 };
+    char const* m_filename { nullptr };
+};
+
+}