rwhod(8): Prevent clobbering the neighboring field by a wrong cast.

rwhod(8): Prevent clobbering the neighboring field by a wrong cast.

The time stamps in the rwhod(8) network datagram and, based on that,
/var/rwho/whod.* spool files are typed as 32bit int and cannot accept
a 64bit time_t.

The old code clobbered the first bytes of the adjacent "hostname"
field, yielding empty hostnames in local rwho(1) and ruptime(1) output.
(Outgoing datagrams to other hosts were not affected.)

Re-code to let the compiler do the proper coercion (well, until 2038).

Submitted-by:  Martin Neitzel
Dragonfly-bug: <https://bugs.dragonflybsd.org/issues/2339>