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/access/nbtree/nbtree.c

@@ -1052,7 +1052,7 @@ restart:
 				opaque->btpo_cycleid == vstate->cycleid)
 			{
 				opaque->btpo_cycleid = 0;
-				MarkBufferDirtyHint(buf);
+				MarkBufferDirtyHint(buf, true);
 			}
 		}