for libthr.so.3, without breaking the ABI. Special value is stored in
the lock pointer to indicate shared lock, and offline page in the shared
memory is allocated to store the actual lock.
Reviewed by: vangyzen (previous version)
Discussed with: deischen, emaste, jhb, rwatson,
Martin Simmons <martin@lispworks.com>
Tested by: pho
Sponsored by: The FreeBSD Foundation