agent: Use MAX_PASSPHRASE_LEN (255) also for the loopback.

* agent/call-pinentry.c (agent_get_passphrase): Reduce maximum
passphrase length as conveyed to the loopback to MAX_PASSPHRASE_LEN.
* agent/genkey.c (agent_ask_new_passphrase): Extend the maximum
passphrase as conveyed to the loopback to MAX_PASSPHRASE_LEN.

Note that in genkey() max_length is set to MAX_PASSPHRASE_LEN + 1
because in agent_askpin() decrements that value before conveying it to
the loopback.

• GnuPG-bug-id: T3254
• Signed-off-by: Werner Koch <wk@gnupg.org>