dhclient: fix time parsing for leases expiring after 2038

Convert lease parsing to timegm to calculate timestamp. For reference, when
writing the lease, we use gmtime to convert the timestamp to struct tm.

Reviewed By: markj, vangyzen
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D40760