billing: Make indexer case insensitive

The billing indexer turns the billing output patterns into regular expressions
and uses those when parsing billing files. Literal text in the pattern are
translated to literal text in the regular expression.

Unfortunately, at some point we altered the capitalization of 'Unknown' in the
output pattern. This causes the billing indexer to fail to match older billing
files.

The patch changes the pattern to case insensitive matching. The generated bloom
filter is still case sensitive.

It is recomended that existing indexes are reindexed using the
'/usr/sbin/dcache-billing-indexer -all' command.

Target: trunk
Require-notes: yes
Require-book: no
Request: 2.8
Request: 2.7
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/6798/
(cherry picked from commit ca3f2447a8a4f2658dd37d94efced5e68f691b78)