boottrace(8): small wrapper utility

This is a small program that when invoked will create start and stop
boottrace entries via sysctl, and execute the desired command. Having
this as an executable -- as opposed to some shell script invoking
sysctl(8) -- allows the total resource usage recorded by the trace
entries to include the child process.

Reviewed by: 0mp, trasz (older version)
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D31929