nixos/acme: replace simp-le with lego client

Lego allows users to use the DNS-01 challenge to validate their
certificates. It is mostly backwards compatible, with a few
caveats.

 - extraDomains can no longer have different webroots to the
   main webroot for the cert.
 - An email address is now mandatory for account creation

The following other changes were required:
 - Deprecate security.acme.certs.<name>.plugins, as this was
   specific to simp-le
 - Rename security.acme.validMin to validMinDays, to avoid
   confusion and errors. Lego requires the TTL to be specified in
   days
 - Add options to cover DNS challenge (dnsProvider,
   credentialsFile, dnsPropagationCheck)
 - A shared state directory is now used (/var/lib/acme/.lego)
   to avoid account creation rate limits and share credentials
   between certs

nixos/modules/security/acme.xml

@@ -7,7 +7,7 @@
  <para>
   NixOS supports automatic domain validation &amp; certificate retrieval and
   renewal using the ACME protocol. This is currently only implemented by and
-  for Let's Encrypt. The alternative ACME client <literal>simp_le</literal> is
+  for Let's Encrypt. The alternative ACME client <literal>LEGo</literal> is
   used under the hood.
  </para>
  <section xml:id="module-security-acme-prerequisites">