ktrace: Increase precision of timestamps.

Replace struct timeval in header with struct timespec.
To differentiate header formats, add a new KTR_VERSIONED flag
set in the header type field similar to the existing KTRDROP flag.

To make it easier to extend ktrace headers in the future,
extend the existing header with a version field (version 0 is
reserved for older records without KTR_VERSIONED) as well as
new fields holding the thread ID and CPU ID.

Reviewed by: jhb, pauamma
Differential Revision: https://reviews.freebsd.org/D35774
MFC after: 2 weeks