checkstyle9.pl: Perl script to check if a change is approximately style(9)

This code is adapted from the QEMU checkpatch.pl script. It can check
either a patch, a file or a git branch. It tries to warn about things
that I believe might be style(9) violations. It's experimental, since I
heavily hacked on the qemu version to get it to not complain (much)
about iconic code in the tree. At the moment, it's use should be
considered expermental. It will likely miss violations, and complain
about code that's perfectly fine. It's offered as an experiment
and to make it easier for contributors to submit patches.