wevdav,xrootd,nfs: Fix blocking reverse DNS lookup

Addresses an issue in these doors that could cause the door to
become unresponsive.

Our monitoring services periodically poll our doors to get a list
of active transfers. The door did a reverse DNS lookup on the client
IP and this lookup could block the door.

Target: trunk
Request: 2.6
Request: 2.5
Request: 2.2
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/5546/
(cherry picked from commit cf364eecc3a2b69e321ad41246c238900fbdf902)
Merge pull request #28 from gbehrmann/fix/2.6/rb5546

wevdav,xrootd,nfs: Fix blocking reverse DNS lookup