dcap: refactor PnfsSessionHandler to unify permission check and url hand...

...ling

fixes issue with permission check with prestage over mounted NFS

Acked-by: Gerd Behrmann
Target: master, 2.11, 2.10
Ticket: #8518, #8532
Require-book: no
Require-notes: no
(cherry picked from commit 14d25b218f633eeba2bb18dcfcebd1d9d3c7f8e0)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>