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ipmux: implement hangup ctl

Implement a hangup ctl command that flushes the
queues, but keeps the filter around.

This can be usefull for low-overhead traffic blocking,
as only the file-descriptor needs to be kept around
and the queues can be flushed.

No user-space process is needed to consume packets
and no buffers are wasted.

example:

aux/dial -e -o hangup 'ipmux!ver=4;src=8.8.8.8' rc -c 'echo 0 > /srv/blocked'

rm /srv/blocked

--- a/sys/src/9/ip/ipmux.c

+++ b/sys/src/9/ip/ipmux.c

@@ -815,6 +815,18 @@

 	return n;

 }

+static char*

+ipmuxctl(Conv *c, char **f, int n)

+{

+	if(n == 1 && strcmp(f[0], "hangup") == 0){

+		/* hangup and flush the queues, but keep the filter around */

+		qclose(c->rq);

+		qclose(c->wq);

+		return nil;

+	}

+	return "unknown control request";

+}

+

 void

 ipmuxinit(Fs *f)

 {

@@ -829,7 +841,7 @@

 	ipmux->create = ipmuxcreate;

 	ipmux->close = ipmuxclose;

 	ipmux->rcv = ipmuxiput;

-	ipmux->ctl = nil;

+	ipmux->ctl = ipmuxctl;

 	ipmux->advise = nil;

 	ipmux->stats = ipmuxstats;

 	ipmux->ipproto = -1;

-- 

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