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)
xrootd: Fix compilation of backported session initialization

Target: 2.11, 2.10
Require-notes: no
Require-book: no
Acked-by: none
(cherry picked from commit 9d1e2728d62cbf36abef623ad8612346a22c9c10)
Merge pull request #1011 from gbehrmann/fix/2.10/rb7437

xrootd: Initialize session