(13051) Make the uniqueid for FreeBSD return the value of sysctl -n kern.hostid

New patch, moves hostuuid to a separate source file and adds unit tests
for both uniqueid and hostuuid on FreeBSD.

This patch was submitted by Garrett Wollman in the description of #13051.
(maint) Document hostuuid and uniqueid facts
(maint) Use sysctl helper method for FreeBSD sysctl calls

Commit fc8b1446c31 added a helper method for cleanly invoking sysctl;
this commit updates the behavior added in bfac93844e to take advantage
of the new method.
Merge branch 'pull-489'

This closes GH-489