Adding kern.features.witness helps expose whether or not the kernel has
`options WITNESS` enabled, so the `feature_present(3)` API can be used
to query whether or not `WITNESS` is built-in.
This support is helpful with userspace applications (generally speaking,
tests), as it can be queried to determine whether or not tests related
to WITNESS should be run.
MFC after: 1 week
Sponsored by: DellEMC Isilon