xrootd: Add access-log plugin

Allows xrootd doors to generate an access log similar to those of SRM, FTP and WebDAV
doors. May optionally be used on pools too.

Target: trunk
Request: 2.11
Request: 2.10
Require-notes: yes
Require-book: no
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Patch: https://rb.dcache.org/r/7442/