we set stack var to 0 even if evlog was off and thus didn't use it.
this cleans up the evlog func a bit and also moves locking until later
so it's locked for the minimum period to punt something into the log
buffer. it's an improvement, but no bug fix.
If we're not logging events this generates a lot of wasted system
calls. They probably don't amount to much, but it's trivial to
get rid of them, and they make a mess when logging with strace.
this checks for clock_gettime + CLOCK_MONOTONIC or CLOCK_REALTIME at
evlog init to avoid a cmp+brang and l1 instr cache hit every get.
slightly less overhead when this is on.