libssh2_set/get_timeout

These functions were added in 1.2.9 of libssh2. I've added them as an attribute of session, akin to blocking. This is done in a way that I hope preserves backward compatibility. I tested with 1.4.2 only.

Benefit of this call is if the channel dies but the session stays active, a ftp_read, say, can hang. Indeterminately. With a timeout set, it will time out.