Add preallocation field to PoolAcceptFileMessage

Adds a field for preallocation to the pool accept message - I imagine the pool
could make use of this information to preallocate space during mover startup or
reject a mover if not enough garbage collectible space is available up front.
In the short term this will be used by space manager for implicit space
reservation.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6386/