src/key-type.{c,h}: Fix a error with double free of memory.

Fix a error with double free of memory.  The problem was that SSH
keys are usually freed along with the object that contains them.
For example, if a key is gotten from a SSH session it will be
freed along with the session.  Not if we try to free the memory
taken by the key then we get "double free" error.  To prevent it
the new field in the key smob structure is introduced to track who
is responsible for freeing the key.

* src/key-type.h (key_data): Add "is_to_be_freed" field.
* src/key-type.c (free_key_smob): Check if the key must be freed.
* src/key-func.c (guile_ssh_private_key_from_file)
  (guile_ssh_public_key_from_private_key)
  (guile_ssh_public_key_from_file): Set "is_to_be_freed" field for
  the key.