nixos/acme: change default keyType to ec256

Previously, the NixOS ACME module defaulted to using P-384 for
TLS certificates. I believe that this is a mistake, and that we
should use P-256 instead, despite it being theoretically
cryptographically weaker.

The security margin of a 256-bit elliptic curve cipher is substantial;
beyond a certain level, more bits in the key serve more to slow things
down than add meaningful protection. It's much more likely that ECDSA
will be broken entirely, or some fatal flaw will be found in the NIST
curves that makes them all insecure, than that the security margin
will be reduced enough to put P-256 at risk but not P-384. It's also
inconsistent to target a curve with a 192-bit security margin when our
recommended nginx TLS configuration allows 128-bit AES. [This Stack
Exchange answer][pornin] by cryptographer Thomas Pornin conveys the
general attitude among experts:

> Use P-256 to minimize trouble. If you feel that your manhood is
> threatened by using a 256-bit curve where a 384-bit curve is
> available, then use P-384: it will increases your computational and
> network costs (a factor of about 3 for CPU, a few extra dozen bytes
> on the network) but this is likely to be negligible in practice (in a
> SSL-powered Web server, the heavy cost is in "Web", not "SSL").

[pornin]: https://security.stackexchange.com/a/78624

While the NIST curves have many flaws (see [SafeCurves][safecurves]),
P-256 and P-384 are no different in this respect; SafeCurves gives
them the same rating. The only NIST curve Bernstein [thinks better of,
P-521][bernstein] (see "Other standard primes"), isn't usable for Web
PKI (it's [not supported by BoringSSL by default][boringssl] and hence
[doesn't work in Chromium/Chrome][chromium], and Let's Encrypt [don't
support it either][letsencrypt]).

[safecurves]: https://safecurves.cr.yp.to/
[bernstein]: https://blog.cr.yp.to/20140323-ecdsa.html
[boringssl]: https://boringssl.googlesource.com/boringssl/+/e9fc3e547e557492316932b62881c3386973ceb2
[chromium]: https://bugs.chromium.org/p/chromium/issues/detail?id=478225
[letsencrypt]: https://letsencrypt.org/docs/integration-guide/#supported-key-algorithms

So there's no real benefit to using P-384; what's the cost? In the
Stack Exchange answer I linked, Pornin estimates a factor of 3×
CPU usage, which wouldn't be so bad; unfortunately, this is wildly
optimistic in practice, as P-256 is much more common and therefore
much better optimized. [This GitHub comment][openssl] measures the
performance differential for raw Diffie-Hellman operations with OpenSSL
1.1.1 at a whopping 14× (even P-521 fares better!); [Caddy disables
P-384 by default][caddy] due to Go's [lack of accelerated assembly
implementations][crypto/elliptic] for it, and the difference there seems
even more extreme: [this golang-nuts post][golang-nuts] measures the key
generation performance differential at 275×. It's unlikely to be the
bottleneck for anyone, but I still feel kind of bad for anyone having
lego generate hundreds of certificates and sign challenges with them
with performance like that...

[openssl]: https://github.com/mozilla/server-side-tls/issues/190#issuecomment-421831599
[caddy]: https://github.com/caddyserver/caddy/blob/2cab475ba516fa725d012f53ca417c3e039607de/modules/caddytls/values.go#L113-L124
[crypto/elliptic]: https://github.com/golang/go/tree/2910c5b4a01a573ebc97744890a07c1a3122c67a/src/crypto/elliptic
[golang-nuts]: https://groups.google.com/forum/#!topic/golang-nuts/nlnJkBMMyzk

In conclusion, there's no real reason to use P-384 in general: if you
don't care about Web PKI compatibility and want to use a nicer curve,
then Ed25519 or P-521 are better options; if you're a NIST-fearing
paranoiac, you should use good old RSA; but if you're a normal person
running a web server, then you're best served by just using P-256. Right
now, NixOS makes an arbitrary decision between two equally-mediocre
curves that just so happens to slow down ECDH key agreement for every
TLS connection by over an order of magnitude; this commit fixes that.

Unfortunately, it seems like existing P-384 certificates won't get
migrated automatically on renewal without manual intervention, but
that's a more general problem with the existing ACME module (see #81634;
I know @yegortimoshenko is working on this). To migrate your
certificates manually, run:

    $ sudo find /var/lib/acme/.lego/certificates -type f -delete
    $ sudo find /var/lib/acme -name '*.pem' -delete
    $ sudo systemctl restart 'acme-*.service' nginx.service

(No warranty. If it breaks, you get to keep both pieces. But it worked
for me.)