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.

headers/private/kernel/vm/vm_page.h

@@ -46,7 +46,7 @@ void vm_page_free_etc(VMCache* cache, vm_page* page,
 	vm_page_reservation* reservation);
 
 void vm_page_set_state(struct vm_page *page, int state);
-void vm_page_requeue(struct vm_page *page, bool tail);
+void vm_page_requeue(struct vm_page *page, bool tail, struct VMPageQueue** _queue);
 
 // get some data about the number of pages in the system
 page_num_t vm_page_num_pages(void);

headers/private/kernel/vm/vm_priv.h

@@ -39,6 +39,8 @@ void vm_debug_init();
 void vm_kernel_args_init_post_area(kernel_args* args);
 status_t vm_daemon_init(void);
 
+bool vm_page_should_do_active_paging();
+
 const char *page_state_to_string(int state);
 	// for debugging purposes only