Fixing wg-quick's DNS= directive with a hatchet

[Jason A. Donenfeld]

Hello everybody,

I have committed the DNS hatchet! But in compromise-form:

https://git.zx2c4.com/WireGuard/commit/?id=f042be244a98a8d854d4b89353190d1673f5f814

Distributions that still need to use it (such as Fedora and OpenSUSE)
can call the apply.sh script in contrib/ to modify things. Everybody
else will keep functioning like before. When no more distributions
need the hatchet, I'll remove it from contrib/. Advantages of this
approach:

- Distributions don't have to maintain it, but still are responsible
for it by virtue of choosing to apply it.
- It doesn't pollute wg-quick.bash itself.
- It's more arguably removable later, since it only lives in contrib/.

So, I'll release a new snapshot soon, and I'll expect to see future
RPMs run apply.sh prior to make install, until they have a more
enterprisey solution available to them.

Glad we worked this out! Thanks everyone for their helpful input.

Regards,
Jason