c/cruft: Use lc + index to optimize license tests

index() is a bit faster than regexes.  This difference accumulates to
a measurable 2 seconds (out of 14s) for source:linux.

At the same time, rewrite the related regexes to be case-sensitive
which should make them slightly more efficient.

Signed-off-by: Niels Thykier <niels@thykier.net>