export used and available space on instance via `xrdfs spaceinfo`

[costing]

Hi,

This is to keep track of the request to expose the top-level counters via the Xrootd protocol, the spaceinfo command in particular.

The call that is expected to work is something like this: xrdfs srm.ndgf.org:1094 spaceinfo /alice/disk

As a reference you can point to an EOS instance, for example with this command: xrdfs eosalice.cern.ch:1094 spaceinfo /eos/alice/cond the output of which is currently:

Path:               /eos/alice/cond
Total:              70000000000000
Free:               68514463763668
Used:               1485536236332
Largest free chunk: 68719476736

Cheers,

.costin