fix for /net.alt

Reference: /n/atom/patch/applied2013/sshsessionnetalt
Date: Sun Nov 17 04:01:35 CET 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/ssh2/sshsession.c	Sun Nov 17 04:01:30 2013
+++ /sys/src/cmd/ssh2/sshsession.c	Sun Nov 17 04:01:31 2013
@@ -17,6 +17,7 @@
 char *nsfile = nil;
 char *uname;
 char *netdir;
+char *netmtpt;
 
 void
 usage(void)
@@ -64,7 +65,16 @@
 	if (uname == nil)
 		uname = "none";
 	netdir = getenv("net");
-	fprint(errfd, "net is %s\n", netdir);
+	if(netdir == nil || *netdir == 0)
+		sysfatal("sshsession: no $net");
+	netmtpt = strdup(netdir);
+	if(netmtpt == nil)
+		sysfatal("sshsession: malloc: %r");
+	p = strchr(netmtpt+1, '/');
+	if(p == nil)
+		sysfatal("sshsession: bad $net: %s", netdir);
+	*p = 0;
+	fprint(errfd, "net is %s; mtpt %s\n", netdir, netmtpt);
 	filnam = smprint("%s/ctl", netdir);
 	topctl = open(filnam, OWRITE);
 	if (topctl < 0) {
@@ -142,7 +152,7 @@
 			fprint(topctl, "hangup");
 			exits(nil);
 		}
-		mount(fd, -1, "/net", MBEFORE, "");
+		mount(fd, -1, netmtpt, MBEFORE, "");
 	}
 	while (1) {
 		fd = open(filnam, ORDWR);