Don't try to shutdown() a pty

shutdown() of a pty doesn't work (ENOTSOCK), so we should close
it instead.

This will ensure that PTY controlling terminals are closed when a
session exits, including when multiple sessions run over a single SSH
connection.  In the normal case of a single session, the PTY controlling
terminal would be closed when the Dropbear server process exits anyway.

This possibly fixes #264 on github

It is possible that there could be subtle changes to PTY flushing
behaviour, though nothing caught by tests at present.

src/channel.h

@@ -55,8 +55,9 @@ struct Channel {
 	int errfd; /* used like writefd or readfd, depending if it's client or server.
 				  Doesn't exactly belong here, but is cleaner here */
 	int bidir_fd; /* a boolean indicating that writefd/readfd are the same
-			file descriptor (bidirectional), such as a network socket or PTY.
-			That is handled differently when closing FDs */
+			file descriptor (bidirectional), such as a network sockets.
+			That is handled differently when closing FDs. Is only
+			applicable to sockets (which can be used with shutdown()) */
 	circbuffer *writebuf; /* data from the wire, for local consumption. Can be
 							 initially NULL */
 	circbuffer *extrabuf; /* extended-data for the program - used like writebuf

src/svr-chansession.c

@@ -910,7 +910,7 @@ static int ptycommand(struct Channel *channel, struct ChanSess *chansess) {
 		channel->readfd = chansess->master;
 		/* don't need to set stderr here */
 		ses.maxfd = MAX(ses.maxfd, chansess->master);
-		channel->bidir_fd = 1;
+		channel->bidir_fd = 0;
 
 		setnonblocking(chansess->master);