webdav: add support for remote copy

Add support for the COPY command for use with third-party copies.  This follows the protocol
described here:

    https://svnweb.cern.ch/trac/lcgdm/wiki/Dpm/WebDAV/Extensions#ThirdPartyCopies

Although the document describes only HTTPS used as a transport, this patch includes a framework
where other transports are supported, with initial support is for gsiftp and https.  The https
support currently fails as that requires another patch.

Target: master