g10: Create keys that expire in simple key generation mode.
* g10/keygen.c (default_expiration_interval): New variable. (generate_keypair): Use the new default.
Cursory discussion on gnupg-devel@ suggested two years as a good
default expiration interval.
- GnuPG-bug-id: T2701
- Signed-off-by: Justus Winter <justus@g10code.com>