Add kern.features.witness

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(9) is built into the kernel.

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
Reviewed by: cem, darrick.freebsd_gmail.com
Differential Revision: https://reviews.freebsd.org/D25302
Sponsored by: DellEMC Isilon