kern.timecounter.choices: Don't offer the dummy counter as an option.
The dummy counter is a stopgap during boot. It is not useful after a
real timecounter is attached and started and there is no reason to return
to using it.
So don't even offer it to the admin. This is easy: never add it to the
timecounter list. It will effectively cease to exist after the first real
timecounter is actived in tc_init().
In principle this means that we can have an empty timecounter list so we
need to check for that case in sysctl_tc_choice().
"I don't mind" mpi@, ok visa@