crypto: Rename check_key_file() -> check_private_key_file().

crypto: Rename check_key_file() -> check_private_key_file().

For public keys the function only called stat(2), which is unnecessary
because only an error from the subsequent open(2) call requires to
fail the operation.

The stat() call is needed for loading private keys though, to make
sure permissions are restrictive enough.

This commit renames the function as indicated in the subject and
drops the second parameter. In crypt.c we now call this function for
private keys only.