channel: Add ssh_channel_accept_forward().

This works same way as ssh_forward_accept() but can return a destination
port of the channel (useful if SSH connection forwarding several TCP/IP
ports).

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
doc: Use ssh_channel_accept_forward() in documentation.
cmake: Increase version numbers for 0.6.1.
bind: fix possible double-frees in ssh_bind_free

Make sure to explicitly set key pointers to NULL following the use
of 'ssh_key_free' throughout bind.c.

Before this change, a double free can happen via 'ssh_bind_free'
as in this example callpath:

  // create an ssh_bind
  ssh_bind b = ssh_bind_new();

  // provide a path to a wrong key-type
  ssh_bind_options_set(b, SSH_BIND_OPTIONS_DSAKEY, path_to_rsa_key);

  // initialize set key-type
  ssh_bind_listen(b);

    -> error path "The DSA host key has the wrong type: %d",

       ssh_key_free(sshbind->dsa)

         -> ssh_key_clean(key) // OK

         -> SAFE_FREE(key)     // OK, but, sshbind->dsa is *not* set to NULL

  // ssh_bind_listen failed, so clean up ssh_bind
  ssh_bind_free(b);

    -> ssh_key_free(sshbind->dsa)  // double-free here

To fix, set pointers to NULL that have been free'd with 'ssh_key_free'.

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