Fix for ssh2_exec(): Unable to request a channel from remote host

We need to set session to blocking mode before calling libssh2_channel_open_session().

This fixes long standing issue of ssh2_exec() failing with 'Unable to request a channel from remote host' when invoked for second time using same session. - https://bugs.php.net/bug.php?id=58893
ssh2_shell() has same construct, so same approach is applied there too.