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.

config/config.go

@@ -419,12 +419,15 @@ type KeyCert struct {
 }
 
 type TLS struct {
-	ACME       string    `sconf:"optional" sconf-doc:"Name of provider from top-level configuration to use for ACME, e.g. letsencrypt."`
-	KeyCerts   []KeyCert `sconf:"optional" sconf-doc:"Key and certificate files are opened by the privileged root process and passed to the unprivileged mox process, so no special permissions are required."`
-	MinVersion string    `sconf:"optional" sconf-doc:"Minimum TLS version. Default: TLSv1.2."`
+	ACME                string    `sconf:"optional" sconf-doc:"Name of provider from top-level configuration to use for ACME, e.g. letsencrypt."`
+	KeyCerts            []KeyCert `sconf:"optional" sconf-doc:"Keys and certificates to use for this listener. The files are opened by the privileged root process and passed to the unprivileged mox process, so no special permissions are required on the files. If the private key will not be replaced when refreshing certificates, also consider adding the private key to HostPrivateKeyFiles and configuring DANE TLSA DNS records."`
+	MinVersion          string    `sconf:"optional" sconf-doc:"Minimum TLS version. Default: TLSv1.2."`
+	HostPrivateKeyFiles []string  `sconf:"optional" sconf-doc:"Private keys used for ACME certificates. Specified explicitly so DANE TLSA DNS records can be generated, even before the certificates are requested. DANE is a mechanism to authenticate remote TLS certificates based on a public key or certificate specified in DNS, protected with DNSSEC. DANE is opportunistic and attempted when delivering SMTP with STARTTLS. The private key files must be in PEM format. PKCS8 is recommended, but PKCS1 and EC private keys are recognized as well. Only RSA 2048 bit and ECDSA P-256 keys are currently used. The first of each is used when requesting new certificates through ACME."`
 
-	Config     *tls.Config `sconf:"-" json:"-"` // TLS config for non-ACME-verification connections, i.e. SMTP and IMAP, and not port 443.
-	ACMEConfig *tls.Config `sconf:"-" json:"-"` // TLS config that handles ACME verification, for serving on port 443.
+	Config                   *tls.Config     `sconf:"-" json:"-"` // TLS config for non-ACME-verification connections, i.e. SMTP and IMAP, and not port 443.
+	ACMEConfig               *tls.Config     `sconf:"-" json:"-"` // TLS config that handles ACME verification, for serving on port 443.
+	HostPrivateRSA2048Keys   []crypto.Signer `sconf:"-" json:"-"` // Private keys for new TLS certificates for listener host name, for new certificates with ACME, and for DANE records.
+	HostPrivateECDSAP256Keys []crypto.Signer `sconf:"-" json:"-"`
 }
 
 type WebHandler struct {