gpg: Improve passphrase caching.

* agent/cache.c (last_stored_cache_key): New.
(agent_get_cache): Allow NULL for KEY.
(agent_store_cache_hit): New.
* agent/findkey.c (unprotect): Call new function and try to use the
last stored key.

* g10/revoke.c (create_revocation): Add arg CACHE_NONCE and pass to
make_keysig_packet.
(gen_standard_revoke): Add arg CACHE_NONCE and pass to
create_revocation.
* g10/keygen.c (do_generate_keypair): Call gen_standard_revoke with
cache nonce.
--

This patch adds two features:

1. The key for the last passphrase successfully used for unprotecting
a key is stored away.  On a cache miss the stored away passphrase is
tried as well.  This helps for the common GPG use case of having a
signing and encryption (sub)key with the same passphrase.  See the
code for more comments.

2. The now auto-generated revocation certificate does not anymore
popup a passphrase prompt.  Thus for standard key generation the
passphrase needs to be given only once (well, two with the
confirmation).

agent/agent.h

@@ -364,6 +364,7 @@ void agent_flush_cache (void);
 int agent_put_cache (const char *key, cache_mode_t cache_mode,
                      const char *data, int ttl);
 char *agent_get_cache (const char *key, cache_mode_t cache_mode);
+void agent_store_cache_hit (const char *key);
 
 
 /*-- pksign.c --*/