Add ldap_pvt_thread_destory() to un-ldap_pvt_thread_initialize().

Move all SunOS routines (ie: _sleep() and its helpers) to thr_lwp.c
so that it's internals can be 'static'.  Add 'static' to thr_lwp.c.
Add comment to LWP:  NEEDS WORK, may not compile!  Need some
SunOS user to sort out the details.  Volunteers?