session: Check the session timeout and use it if set

This checks if a timeout has been set using ssh_options_set(). If it has
been set it will use that parametr by default for blocking sessions.

This is at least what users are expecting.

Fixes T33

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

src/channels.c

@@ -2751,7 +2751,7 @@ int ssh_channel_read_timeout(ssh_channel channel,
   ctx.buffer = stdbuf;
   ctx.count = 1;
 
-  if (timeout_ms < 0) {
+  if (timeout_ms < SSH_TIMEOUT_DEFAULT) {
       timeout_ms = SSH_TIMEOUT_INFINITE;
   }