pw(8): r326738 broke expiration arithmetic in case of `-D' flag not supplied.

Fix it and rename misnamed time_t variables `expire_days, password_days'
(always holding absolute time) to `expire_time, password_time'.

Add a comment for a case of overloading `cmdcnf->password_days'
and `cmdcnf->expire_days' with absolute time.

Reported by: markj
Approved by: mav (mentor)
MFC after: 1 week