ntptime: Use time_t for tv_sec related variables

The struct timespec tv_sec member is of type time_t. Make sure that all
variables related to this member are of the type time_t. This is important for
targets where long is a 32-bit type and time_t a 64-bit type.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1373