Add buffer_std flag to MarkBufferDirtyHint().

MarkBufferDirtyHint() writes WAL, and should know if it's got a
standard buffer or not. Currently, the only callers where buffer_std
is false are related to the FSM.

In passing, rename XLOG_HINT to XLOG_FPI, which is more descriptive.

Back-patch to 9.3.

src/backend/storage/freespace/fsmpage.c

@@ -284,7 +284,7 @@ restart:
 				exclusive_lock_held = true;
 			}
 			fsm_rebuild_page(page);
-			MarkBufferDirtyHint(buf);
+			MarkBufferDirtyHint(buf, false);
 			goto restart;
 		}
 	}