kernel/vm: Split the page writer into its own file and queue subclass.

This paves the way for there being more than one modified queue,
e.g. for each KDiskDevice.

src/system/kernel/cache/file_cache.cpp

@@ -890,7 +890,7 @@ do_cache_io(void* _cacheRef, void* cookie, off_t offset, addr_t buffer,
 			if (page->State() == PAGE_STATE_CACHED
 					|| page->State() == PAGE_STATE_MODIFIED) {
 				DEBUG_PAGE_ACCESS_START(page);
-				vm_page_requeue(page, true);
+				vm_page_requeue(page, true, NULL);
 				DEBUG_PAGE_ACCESS_END(page);
 			}