MINOR: stream-int: rename SI_FL_WAIT_ROOM to SI_FL_RXBLK_ROOM

This flag is not enough to describe all blocking situations, as can be
seen in each case we remove it. The muxes has taught us that using multiple
blocking flags in parallel will be much easier, so let's start to do this
now. This patch only renames this flags in order to make next changes more
readable.
diff --git a/src/dns.c b/src/dns.c
index 90a937c..4ceda4d 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -2030,7 +2030,7 @@
 			/* let's try again later from this session. We add ourselves into
 			 * this session's users so that it can remove us upon termination.
 			 */
-			si->flags |= SI_FL_WAIT_ROOM;
+			si->flags |= SI_FL_RXBLK_ROOM;
 			return 0;
 		}
 		/* fall through */