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 parameter by default for blocking reads.

This is at least what users are expecting.

Fixes T33

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>