this will be useful for converting some of the nanosleep calls into
scrotSleepFor().

absolute time is really awkward to use. scrotSleepFor() instead takes a
starting timestamp and a relative millisecond offset from that start and
returns back the ending timestamp.

this, combined with clockNow(), makes the API easy to use and easy to
replace the existing nanosleep calls with.

avoids potential multiplication overflow when the seconds are converted
into milliseconds.