xrootd: add auth request together with unix auth

Created by: EgorMatirov

Updates #170. Updates #250.

I have tested it locally against the XRootD server with such config:

ofs.trace all
xrd.trace all
#sec.level all pedantic force
xrootd.seclib /usr/lib/libXrdSec.so
sec.protocol /usr/lib/ unix
sec.protocol /usr/lib/ pwd
sec.protbind * unix pwd
acc.authdb /tmp/Authfile
ofs.authorize 1

/tmp/Authfile is:

u ematirov /tmp a
u DESKTOP-R8CLF5C\ematirov /tmp a

Note that even Windows is supported by unix auth. :-P