patch review: import tslog(4) event tracing facility from FreeBSD

[Emile `iMil' Heitor <imil%home.imil.net@localhost>]

This patch https://imil.net/NetBSD/tslog.patch is a port of FreeBSD's
tslog(4) which permits to ease pinpointing parts of the kernel and
userland where the most time is spent.

This tracing facility is what allowed to reduce NetBSD kernel boot
time as it gives the ability to produce flamegraphs like this one
https://imil.net/NetBSD/tslog-10052025.svg and see what functions are
taking the most time.

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Emile `iMil' Heitor <imil@{home.imil.net,NetBSD.org}> | https://imil.net