delegation: add support for GridSite

This patch adds support for GridSite delegation v2.0.0.  The endpoint is
run as part of the SRM service and has the path /srm/delegation and
allows a user to delegate a credential, discover the remaining lifetime
of any delegated credential and destroy delegated credentials.  Those
credentials delegated via SRM and httpg have the delegated ID of 'gsi'.
Using this, a client may delete credentials delegated via SRM
operations.

Also included is a delegation client that allows scripted and
console-based interaction with GridSite endpoints.

Target: master
Requires-notes: true
Requires-book: true
Patch: http://rb.dcache.org/r/6628
Acked-by: Gerd Behrmann