lib/thread: Prune a little more

lib/thread: Prune a little more

Remove automatic VThread_SetName for unmanaged threads.

Other minor removals and tweaks:
- modify "safe" name in VThreadBase to use gettid or
  pthread_threadid_np if available
- Move lib/thread name computation to child thread to remove dependency
  on VThreadID in parent.
- Convert VThreadFreeID to VThreadFree (and avoid a lookup)
- Move VThreadBase_ForgetSelf to VThreadDestroyThread to
  make VThreadHostDestroyThread straightforward.
- The 'priority' field had become write-only after earlier cleanups
  to replace AdjustThreadPriority with SetThreadPriority. Remove it,
  and some dead comments mentioning it.
- Remove unused 'signalNestCount' atomic.
- Remove two unimplemented VThreadInt prototypes.
- Remove a vacuously-true ASSERT in VThreadHostExitThread()