[skip ci] add a CODEOWNERS file

Summary:
Convert MAINTAINERS into a Github CODEOWNERS file. This will
automatically assign reviewers to some GH pull requests. The conversion
is not 1:1; some committers don't have Github accounts (e.g. adrian),
some functional areas don't neatly correspond to a set of files (e.g.
kqueue), and mailing lists can't be assigned as a reviewer (e.g.
secteam@). But it's a start.

MFC after: 2 weeks
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D30559