agent: New option --need-attr for KEYINFO.

* agent/command.c (do_one_keyinfo): New arg need_Attr.
(cmd_keyinfo): New option --need-attr.
* agent/findkey.c (public_key_from_file): Use nvc_get_boolean.

This option makes it easier to list keys suitable only for certain
purposes.

The second patch makes if compliant to the description in
keyformat.txt