agent: New option --restricted for PRESET_PASSPHRASE.

* agent/command.c (cmd_preset_passphrase): Add option.
* agent/preset-passphrase.c (oRestricted): New.
(opts): Add option --restricted.
(main): Set option.
(preset_passphrase): Use option.

We use a different cache for connections from the extra-socket.
However, with gpg-preset-passphrase is only able to preset a
passphrase into the regular cache. Further, a restricted connection
may not use PRESET_PASSPHRASE. To solve this we add an new option to
preset the passphrase into the "restricted" cache. For the
gpg-preset-passphrase tool we also add the option --restricted.

Note that this does not yet work with gpg-preset-passphrase --forget.