[kernel] Add VMA interface

Finished the VMA kobject and added the related syscalls. Processes can
now allocate memory! Other changes in this commit:

- stop using g_frame_allocator and add frame_allocator::get()
- make sure to release all handles in the process dtor
- fix kutil::map::iterator never comparing to end()

src/kernel/frame_allocator.h

@@ -29,6 +29,9 @@ public:
 	/// \arg count    The number of frames to be freed
 	void free(uintptr_t address, size_t count);
 
+	/// Get the global frame allocator
+	static frame_allocator & get();
+
 private:
 	frame_block_list m_free; ///< Free frames list