Get rid of the semaphore implementation, no need to carry both.
Add different locking modes (exclusive and readwrite) to enable
a wider range of testing. Also combine lockfile and lockfile_batch,
the latter is now a postfix option to the former.
So to enable readers-excluding-writers locking mode with a lock batch
count of 4, you would write:
lockfile=readwrite:4
instead.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>