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.

In contrast to 2.12, the plugin is not enabled by default.

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/
(cherry picked from commit 25250c09e8288b18ee4a0a569676a9a9db51e3b1)