quickstart: check if domain was registered recently, and warn about potential deliverability issues

we use 6 weeks as the cutoff, but this is fuzzy, and will vary by mail
server/service provider.

we check the domain age using RDAP, the replacement for whois. it is a
relatively simple protocol, with HTTP/JSON requests. we fetch the
"registration"-related events to look for a date of registration.
RDAP is not available for all country-level TLDs, but is for most (all?) ICANN
global top level domains. some random cctlds i noticed without rdap: .sh, .au,
.io.

the rdap implementation is very basic, only parsing the fields we need. we
don't yet cache the dns registry bootstrap file from iana. we should once we
use this functionality from the web interface, with more calls.

doc.go

@@ -110,6 +110,7 @@ any parameters. Followed by the help and usage information for each command.
 	mox dnsbl check zone ip
 	mox dnsbl checkhealth zone
 	mox mtasts lookup domain
+	mox rdap domainage domain
 	mox retrain [accountname]
 	mox sendmail [-Fname] [ignoredflags] [-t] [<message]
 	mox spf check domain ip
@@ -186,7 +187,7 @@ output of "mox config describe-domains" and see the output of
 	  -hostname string
 	    	hostname mox will run on, by default the hostname of the machine quickstart runs on; if specified, the IPs for the hostname are configured for the public listener
 	  -skipdial
-	    	skip check for outgoing smtp (port 25) connectivity
+	    	skip check for outgoing smtp (port 25) connectivity or for domain age with rdap
 
 # mox stop
 
@@ -1456,6 +1457,24 @@ should be used, and how long the policy can be cached.
 
 	usage: mox mtasts lookup domain
 
+# mox rdap domainage
+
+Lookup the age of domain in RDAP based on latest registration.
+
+RDAP is the registration data access protocol. Registries run RDAP services for
+their top level domains, providing information such as the registration date of
+domains. This command looks up the "age" of a domain by looking at the most
+recent "registration", "reregistration" or "reinstantiation" event.
+
+Email messages from recently registered domains are often treated with
+suspicion, and some mail systems are more likely to classify them as junk.
+
+On each invocation, a bootstrap file with a list of registries (of top-level
+domains) is retrieved, without caching. Do not run this command too often with
+automation.
+
+	usage: mox rdap domainage domain
+
 # mox retrain
 
 Recreate and retrain the junk filter for the account or all accounts.