xrootd: Initialize session

Initializes a session string for xrootd.

Target: master
Request: 2.11
Request: 2.10
Require-notes: yes
Require-book: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7437/
(cherry picked from commit 2fda3350c67486f871a1c22e73e6f2f23e380596)