client: use ssh_channel_do_free in ssh_disconnect

Ensure to use 'ssh_channel_do_free' in 'ssh_disconnect', when removing and
free'ing up a session's channels.  This matches the behavior in 'ssh_free',
and is necessary to fully free any channel which may not have been closed
completely (see usage of flags SSH_CHANNEL_FLAG_CLOSED_REMOTE,
SSH_CHANNEL_FLAG_FREED_LOCAL).