XXX This must not be enabled by default because the LWP private mechanism
is reserved for TLS. It is provided only as a test/demo.
XXX Since ucontext_t does not contain the thread private variable, for a
short time after threads are created their thread specific data is unset.
If a signal arrives during that time we are screwed.
- No longer need pthread__osrev.
- Rearrange _lwp_ctl() calls slightly.
5c670ea686