Add ability to adjust posix clock_gettime CLOCK_.  Document `ns` better. Add a <64-bit check to `ns` just in case some poor soul tries to use this on a 32-bit posix platform.  Add convenience call `getTmNs` that also has a clockId parameter.
Add ability to adjust posix clock_gettime CLOCK_.  Document `ns` better.
Add a <64-bit check to `ns` just in case some poor soul tries to use this
on a 32-bit posix platform.  Add convenience call `getTmNs` that also has
a clockId parameter.

c-blake/cligen