Fix #175: mrdisc_deregister() fails for never started vif

When first starting up SMCRoute with multiple interfaces, where at least
one phyint does *not* have mrdisc enabled, smcrouted fails with "parse
error" because mrdisc_deregister() fails to deregister a never before
registered mridsc interface.

This patch drops the bogus error, and adds a TODO entry for refactoring
the mrdisc support -- not just for this problem, but preparing for IPv6
and multi-platform support.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/TODO.md

@@ -1,3 +1,13 @@
+
+Refactor MRDISC Support
+-----------------------
+
+The current MRDISC implementation is fragile (see issue #175 for an
+example), and it also does not work on non-Linux systems.  So the
+implementation really needs to be refactored, not just for this but also
+for adding IPv6 support (below).
+
+
 Add Support for IPv6 MRDISC
 ---------------------------
 

src/mrdisc.c

@@ -325,8 +325,7 @@ int mrdisc_deregister(short vif)
 		return 0;
 	}
 
-	errno = ENOENT;
-	return -1;
+	return 0;
 }
 
 void mrdisc_send(void *arg)