pkgs: Debian package missing chown for alarms directory

The debian packaging was missing the chown statement for the /var/lib/dcache/alarms directory; consequently it was being created by root.

Patch adds necessary line to postinst.template.

Target: trunk
Request: 2.6
Patch: http://rb.dcache.org/r/5492/
Bug: http://rt.dcache.org/Ticket/Display.html?id=7774
Require-notes: yes
Require-book: no
Acked-by: Gerd

RELEASE NOTES:

Fixes bug where alarms directory is owned by root instead of dcache user (Debian).