watcher: Port Change notifications list domains apart from hostnames (closes #248)

A Port Change notification's `Hosts:` line listed a port's apex domains
and hostnames together. It now has a `Domains:` line and a `Hostnames:`
line, and leaves out one that would name nothing. A name is a domain
when it is a configured domain, the rule record notifications already
use to start `Domain:` or `Hostname:`; that rule is now one method,
isDomain, which both use. The port entries saved in the state are
unchanged. README describes the new lines.

Model: opus-5-5

README.md

@@ -201,7 +201,9 @@ includes:
 - **NS recoveries**: Which nameserver recovered, which hostname/domain.
 - **NS inconsistencies**: Which nameservers disagree, what each one returned,
   which hostname or domain affected.
-- **Port changes**: Which IP:port, its new state, all associated hostnames.
+- **Port changes**: Which IP:port, its new state, and the domains and the
+  hostnames that resolve to it, on a `Domains:` line and a `Hostnames:` line. A
+  line that would name nothing is left out.
 - **TLS expiry warnings**: Expiry date and days remaining, CN, associated
   hostname and IP.
 - **TLS certificate changes**: Old and new CN and issuer, associated hostname