Screen keyserver responses.

* g10/main.h: Typedef import_filter for filter callbacks.
* g10/import.c (import): Add filter callbacks to param list.
(import_one): Ditto.
(import_secret_one): Ditto.
(import_keys_internal): Ditto.
(import_keys_stream): Ditto.
* g10/keyserver.c (keyserver_retrieval_filter): New.
(keyserver_spawn): Pass filter to import_keys_stream()

These changes introduces import functions that apply a constraining
filter to imported keys. These filters can verify the fingerprints of
the keys returned before importing them into the keyring, ensuring that
the keys fetched from the keyserver are in fact those selected by the
user beforehand.

Re-indention and minor changes by wk.

• Signed-off-by: Stefan Tomanek <tomanek@internet-sicherheit.de>