* agent/agent.h (agent_askpin): Add arguments keyinfo and cache_mode.
Update callers.
(agent_get_passphrase): Likewise.
(agent_clear_passphrase): New function.
(opt): Add field allow_external_cache.
* agent/call-pinentry.c (start_pinentry): Send "OPTION
allow-external-password-cache" to the pinentry.
(PINENTRY_STATUS_PASSWORD_FROM_CACHE): New constant.
(pinentry_status_cb): New function.
(agent_askpin): Add arguments keyinfo and cache_mode.  If KEYINFO and
CACHE_MODE describe a cachable key, then send SETKEYINFO to the
pinentry.  Pass PINENTRY_STATUS_CB to the "GETPIN" invocation.  If the
passphrase was incorrect and PINENTRY_STATUS_PASSWORD_FROM_CACHE is
set, decrement PININFO->FAILED_TRIES.
(agent_get_passphrase): Add arguments keyinfo and cache_mode.  If
KEYINFO and CACHE_MODE describe a cachable key, then send SETKEYINFO
to the pinentry.
(agent_clear_passphrase): New function.
* agent/call-pinentry.c (start_pinentry): Act upon new var,
allow_external_cache.
* agent/command.c (cmd_clear_passphrase): Call agent_clear_passphrase.
* agent/gpg-agent.c (oNoAllowExternalCache): New.
(opts): Add option --no-allow-external-cache.
(parse_rereadable_options): Set this option.