gpg: Fix a memory leak in batch key generation

* g10/keygen.c (append_to_parameter): New.
(proc_parameter_file): Use new func to extend the parameter list.
* g10/passphrase.c (passphrase_to_dek_ext): Print a diagnostic of
gcry_kdf_derive failed.
* g10/keygen.c (proc_parameter_file): Print a diagnostic if
passphrase_to_dek failed.

Due to an improper way of using the linked list head, all memory for
items allocated in proc_parameter_file was never released. If batched
key generation with a passphrase and more than ~200 keys was used this
exhausted the secure memory.