Re-add missing features in multisource code

Two features were found to be missing in the multisource code:

1) Sending monitoring data to the xrootd server (this allows the Dashboard to associate Xrootd activity with a particular grid job).  This was a regression from the prior XrdFile.
2) IPv6 support.  This was accidentally disabled because ROOT loads XrdClient after loading XrdCl.

This pull requests adds support