For consistency, specify the nanosecond option in the same way as other
options, and identify it as such. Split the detection of platform
support (`FindStatNsec`) and its selection (`SelectNsec`).
Threading can now be disabled with `USE_THREADS=OFF` instead of
`THREADSAFE=OFF` to better support the other cmake semantics.
Nanosecond support is the default _if_ we can detect it. This should be
our default always - like threads - and people can opt out explicitly.
Our custom CMake module currently live in "cmake/Modules". As the
"cmake/" directory doesn't contain anything except the "Modules"
directory, it doesn't really make sense to have the additional
intermediate directory. So let's instead move the modules one level up
into the "cmake/" top level directory.
