[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.cpp

@@ -18,6 +18,13 @@ frame_block::compare(const frame_block &rhs) const
 }
 
 
+frame_allocator &
+frame_allocator::get()
+{
+	extern frame_allocator &g_frame_allocator;
+	return g_frame_allocator;
+}
+
 frame_allocator::frame_allocator() {}
 
 size_t