Throttle plugin - groundwork

These two patches are necessary for the throttle plugin to function.

The first adds a new prctl to hint to the underlying SFS a read will be done that bypasses the SFS (i.e., a sendfile call).  This is necessary for the throttle to keep proper accounting.  It also adds the proper handling of a redirect on read, including monitoring.

The second is just a few Atomics fixups; the throttle plugin requires a proper return value from atomic operations.