Implement sysctl kern.boot_id

Boot IDs are random, opaque 128-bit identifiers that distinguish distinct
system boots. A new ID is generated each time the system boots. Unlike
kern.boottime, the value is not modified by NTP adjustments. It remains fixed
until the machine is restarted.

PR: 244867
Reported by: Ricardo Fraile <rfraile AT rfraile.eu>
MFC after: I do not intend to, but feel free