chpass: reject change/expiry dates beyond y2106

The pwd.db and spwd.db files store the change and expire dates as
unsigned 32-bit ints, which overflow in 2106. Reject larger values for
now, until the introduction of a v5 password database.

i386 has 32-bit time_t and so dates beyond y2038 are already rejected by
mktime.

PR: 227589
Reviewed by: lidl
MFC after: 1 week
Sponsored by: The FreeBSD Foundation