In pgbench logging, avoid assuming that instr_times match Unix timestamps.

For aggregated logging, pg_bench supposed that printing the integer part of
INSTR_TIME_GET_DOUBLE() would produce a Unix timestamp.  That was already
broken on Windows, and it's about to get broken on most other platforms as
well.  As in commit 74baa1e3, we can remove the entanglement at the price
of one extra syscall per transaction; though here it seems more convenient
to use time(NULL) instead of gettimeofday(), since we only need
integral-second precision.

I took the time to do some wordsmithing on the documentation about
pgbench's logging features, too.

Discussion: https://postgr.es/m/8837.1483216839@sss.pgh.pa.us