billing: Add -f option to dcache billing command

The patch adds the -f option to the find command of the billing indexer. The
option behaves like the -f option of grep, allowing search terms to be loaded
from a file.

Also extends the existing find command with the ability to accept multiple
search terms at once. This allows more efficient searching than running the
search several times.

Target: trunk
Require-notes: yes
Require-book: yes
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6836/