implement dnssec-awareness throughout code, and dane for incoming/outgoing mail delivery

the vendored dns resolver code is a copy of the go stdlib dns resolver, with
awareness of the "authentic data" (i.e. dnssec secure) added, as well as support
for enhanced dns errors, and looking up tlsa records (for dane). ideally it
would be upstreamed, but the chances seem slim.

dnssec-awareness is added to all packages, e.g. spf, dkim, dmarc, iprev. their
dnssec status is added to the Received message headers for incoming email.

but the main reason to add dnssec was for implementing dane. with dane, the
verification of tls certificates can be done through certificates/public keys
published in dns (in the tlsa records). this only makes sense (is trustworthy)
if those dns records can be verified to be authentic.

mox now applies dane to delivering messages over smtp. mox already implemented
mta-sts for webpki/pkix-verification of certificates against the (large) pool
of CA's, and still enforces those policies when present. but it now also checks
for dane records, and will verify those if present. if dane and mta-sts are
both absent, the regular opportunistic tls with starttls is still done. and the
fallback to plaintext is also still done.

mox also makes it easy to setup dane for incoming deliveries, so other servers
can deliver with dane tls certificate verification. the quickstart now
generates private keys that are used when requesting certificates with acme.
the private keys are pre-generated because they must be static and known during
setup, because their public keys must be published in tlsa records in dns.
autocert would generate private keys on its own, so had to be forked to add the
option to provide the private key when requesting a new certificate. hopefully
upstream will accept the change and we can drop the fork.

with this change, using the quickstart to setup a new mox instance, the checks
at internet.nl result in a 100% score, provided the domain is dnssec-signed and
the network doesn't have any issues.

gendoc.sh

@@ -3,17 +3,9 @@
 (
 cat <<EOF
 /*
-Command mox is a modern full-featured open source secure mail server for
+Command mox is a modern, secure, full-featured, open source mail server for
 low-maintenance self-hosted email.
 
-  - Quick and easy to set up with quickstart and automatic TLS with ACME and
-    Let's Encrypt.
-  - IMAP4 with extensions for accessing email.
-  - SMTP with SPF, DKIM, DMARC, DNSBL, MTA-STS, TLSRPT for exchanging email.
-  - Reputation-based and content-based spam filtering.
-  - Internationalized email.
-  - Admin web interface.
-
 # Commands
 
 EOF