ssh/key.scm (private-key-to-file): New procedure

* ssh/key-func.c (guile_ssh_private_key_to_file): New procedure.
* ssh/key.scm (private-key-to-file): Export.
* tests/key.scm ("private-key-to-file"): New TC.
* doc/api-keys.texi (Keys): Add description of `private-key-to-file'.
* NEWS: Update.