lib: Move libopencsd to /usr/lib and make private

libopencsd is used to decode Arm CoreSight traces. It isn't needed
during the boot so can be moved to /usr/lib, and may change it's API
so can be marked as private as onlt the future hwt tool will use it.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D51380