export space usage information via the `xrdfs query space` command

[costing]

Hi,

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

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

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

oss.cgroup=default&oss.space=70000000000000&oss.free=68514463763668&oss.maxf=68719476736&oss.used=1485536236332&oss.quota=70000000000000

Cheers,

.costin