Add debug helper functions to mark pages present.

They can be used to mark pages as present/non-present without actually
unmapping them. Marking pages as non-present causes every access to
fault. We can use that for debugging as it allows us to "read protect"
individual kernel pages.

headers/private/kernel/vm/VMTranslationMap.h

@@ -36,6 +36,9 @@ struct VMTranslationMap {
 									vm_page_reservation* reservation) = 0;
 	virtual	status_t			Unmap(addr_t start, addr_t end) = 0;
 
+	virtual	status_t			DebugMarkRangePresent(addr_t start, addr_t end,
+									bool markPresent);
+
 	// map not locked
 	virtual	status_t			UnmapPage(VMArea* area, addr_t address,
 									bool updatePageQueue) = 0;