Read the clock once when scheduling a check for a given time

[nook24]

t-tap/test_commands fails at random, about once in 40 runs, in these four assertions:

not ok 436 - SCHEDULE_HOST_SVC_CHECKS schedules host service checks
not ok 473 - SCHEDULE_FORCED_HOST_SVC_CHECKS schedules forced checks for services on a host
not ok 495 - SCHEDULE_HOST_CHECK schedules a host check
not ok 497 - SCHEDULE_FORCED_HOST_CHECK schedules a host check

schedule_service_check() and schedule_host_check() turn check_time into a delay with time(NULL), and schedule_next_*_check() turns it back into next_check with gettimeofday(). On Linux time() reads the coarse clock, which lags behind until the next timer tick. For a few milliseconds after every second boundary it still shows the previous second (0.29% of the time in a tight loop here). A call in that window schedules the check one second later than asked for.

Both functions now read the clock once and pass it to a static helper. The public signatures in checks_service.h and checks_host.h are unchanged. Outside the tests the effect is small: a check scheduled for a fixed time through SCHEDULE_SVC_CHECK and friends landed a second late about 0.3% of the time.