.github: Narrow sys/crypto warning to sys/crypto/skein

This is the only subdirectory of sys/crypto that is a vendor import.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D54719

github: Add more context to pull requests

Add code to fetch and decode CODEOWNERS and automatically @ people in
the review.

Create a new file, .github/path-rules.txt that has a set of paths to
matc

github: Add more context to pull requests

Add code to fetch and decode CODEOWNERS and automatically @ people in
the review.

Create a new file, .github/path-rules.txt that has a set of paths to
match and specific warnings about that part of the tree. We'll use this
to try to wave off pull requests in certain areas of the tree, as well
as remind people when contrib is involved, etc.

Sponsored by: Netflix

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