Improve working with public keys represented as a SSH string.

* src/key-type.h: Improve storing of public keys represented as a
  SSH string -- store the key type along with the key itself.
* src/key-func.c (guile_ssh_public_key_from_file): Likewise.
  (public_key_to_ssh_string): Update.
* src/key-type.c (guile_ssh_key_get_type): New function.
  (scm_from_ssh_key_type): New static function.
  (free_key_smob): Update.
  (init_key_type): Define ssh:get-key-type.
* src/key.scm: Export ssh:get-key-type.