Add option lock/nolock
Add option lock/nolock

Option lock (default) uses a rwlock to manage
resource state (position, etc.)  However,
if the mmap resource is only to be used from
one process, option 'nolock' can be used to
avoid locking operations.
saleyn/emmap,krestenkrab/emmap,saleyn/emmap,saleyn/emmap,krestenkrab/emmap