libc/time/lib_localtime.c is much closer to NuttX coding style. libc/time/private.h removed; incorporated into lib_localtime.c

When reading the system timer, don't read a struct timespec, convert it to a fake tick count, then back to a timespec.  Remove CLOCK_ACTIVETIME.. it is non-standard, unused, and gets in the way of changes like this

Tickless Stuff:  Back out the risky timer operations when the ready-to-run list is modified.  That is unsafe.  An ugly workaround is just to keep an interval timer going all of the time with a minimum duration equal to the timeslice interval.

Timers: In Tickless mode, need to stop the interval timer before inserted a new delay into the timer list.  Otherwise, the time is incorrect on the first entry of the list