If a specific timecounter has been chosen via sysctl, and a new
timecounter with higher quality registers (presumably in a module that has
just been loaded), do not undo the user's choice by switching to the new
timecounter.
Document that behavior, and also the fact that there is no way to
unregister a timecounter (and thus no way to unload a module containing
one).
8cf9fedbf6