Correct test to see if a subscriber has an IPv4 address. This caused
locate to fail when being requested by an IPv4 client.
Differentiate between a locate failure because the file was not found
or that none of the servers were IPv4 reachable.
Add support for returning a hostname in a locate response. This
only implements this now in xrootd and is ignored by the cmsd.
Full support awaits longer message buffers.