[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/objects/process.cpp

@@ -32,6 +32,8 @@ process::process(page_table *kpml4) :
 
 process::~process()
 {
+	for (auto &it : m_handles)
+		if (it.val) it.val->handle_release();
 	s_processes.remove_swap(this);
 }