webadmin: alarms -- fix behavior of display table header checkboxes

module: webadmin

The header checkboxes on the alarm display table have a custom jquery/javascript executable which allows them to set or unset all the visible checkboxes in the given table column.  However, when refresh is activated, the checkboxes remain checked, which is annoying.

This patch implements a clearHeader method which programmatically clears the header checkboxes after refresh.

Target: master
Acked-by: Dmitry
Patch: http://rb.dcache.org/r/5552/
Require-notes: yes
Require-book: no
Request: 2.6