Implement issue 13433 - add option for using coarser realtime clock.

This adds an option for getting the time using the coarser, faster
realtime clock on systems that support it for those who need to get the
time frequently (and thus quickly) but don't need it to be particularly
precise.

https://issues.dlang.org/show_bug.cgi?id=13433

The OSX change is because it's the only one using `gettimeofday`. I figured that it was better to change that to go off of the OS version rather than whether `clock_gettime` existed or not.
+1. However, I remember some time ago that somebody objected to `std.typecons.Flag` for some reason I can't remember... nevertheless, I agree we should use it, it does make the code a lot clearer.