fix handling of DN with comma as per rfc1779

Ticket: http://rt.dcache.org/Ticket/Display.html?id=8285
Ticket: http://rt.dcache.org/Ticket/Display.html?id=8312
(cherry picked from commit ee4180cf1a845990de7b7eb6fba013c6dd5ffc1c)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>