Add multisource version of XrdAdaptor; based on the prior dev branch at https://github.com/bbockelm/XrdAdaptor.
Re-enable setting of debug level.
Remove use of private Xrootd headers.
Per review comments, remove usage of clock_gettime on OS X.
Fix other uses of CLOCK_MONOTONIC.
No longer force a new stat to be performed on an open file;
this fails on EOS.

Fixup error messages to use the correct ToStr.
Keep a copy of the original exception info and append it to the
fallback file-open failure message.
Merge pull request #558 from bbockelm/multisource-xrootd

Multisource xrootd