Import wireguard-tools for wg(8)

744bfb213144 ("Import the WireGuard driver from zx2c4.com") re-imported
the WireGuard driver with the intention that wg(8) will be used to
manage WireGuard interfaces, as on other platforms, now that wg(8) has
been dual-licensed MIT specifically to allow our use in base (thanks!).

This is a copy of wireguard-tools/src, with files that we don't need
.gitignore'd out to make it more clear that we're only building files
that are either MIT or dual-licensed MIT. We may go with a different
structure later (e.g., if we end up needing to include outside of src/),
but an upstream restructure seems unlikely in the foreseeable future.