src/auth.c: Implement ssh:userauth-none!

* src/auth.c (guile_ssh_userauth_none): New function.
  (ssh_auth_result_to_symbol): New static function.
  (guile_ssh_userauth_pubkey): Use ssh_auth_result_to_symbol to
  convert auth result to a Scheme symbol.
  (guile_ssh_userauth_password): Likewise.
* src/auth.scm: Export ssh:userauth-none!