pool: Make xrootd max frame size configurable

Addresses the issue that the xrootdMoverMaxFrameSize property was
ignored by the xrootd mover.

Target: trunk
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/5622/
(cherry picked from commit da723b40be7477b0568769cdc25020425d9acacf)