Make "matches" case-insensitive.

Make the "matches" operator case-insensitive by default. Case
sensitivity can be switched back on using "(?-i)".

It might be nice to make "contains" case-insensitive as well, but we'd
need a caseless version of epan_memmem.

Change-Id: I5e39a52c148477c30c808152bcace08348df815a
Reviewed-on: https://code.wireshark.org/review/22330


Reviewed-by: Gerald Combs <gerald@wireshark.org>
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>