Fix for stacking locks in brlock and POSIX. Windows only allows a read lock

Fix for stacking locks in brlock and POSIX. Windows only allows a read lock
to overlay a write lock on the same fnum. When overlaying read locks onto
a write lock, the number of locks is counted, and the first unlock removes
the write lock and downgrades this to a read lock. Do the same when mapping
to POSIX.
Jeremy.
(This used to be commit 74d42644e6e52808037975e909aa56c850838b76)