* options.h, g10.c (main): Add new --default-sig-expire and
--default-cert-expire options.  Suggested by Florian Weimer.
* main.h, keygen.c (parse_expire_string, ask_expire_interval): Use
defaults passed in, or "0" to control what default expiration is.
* keyedit.c (sign_uids), sign.c (sign_file, clearsign_file,
sign_symencrypt_file): Call them here, so that default expiration
is used when --ask-xxxxx-expire is off.