improve quickstart and readme for better first-time experience

- make the example commands in the readme more likely to succeed, especially
  for people who are not familiar with go and its toolchain.
- improve probability that the correct configuration is generated, especially
  the hostname. previously, if the quickstart email address was "some.domain",
  and the machine where you ran the quickstart was "myhost", the hostname used
  for the configuration was assumed to be "myhost.some.domain". but this is often
  not correct, especially when configuring mox to serve mail on a subdomain of an
  existing domain. mox will now try to determine the host name by a reverse
  lookup of the public ips it found. and it will warn if there are no/multiple
  candidates.

based on feedback from erik dubbelboer, thanks!

mox-/admin.go

@@ -486,6 +486,8 @@ func DomainRecords(domConf config.Domain, domain dns.Domain) ([]string, error) {
 		"",
 		// ../rfc/6186:242
 		"; Next records specify POP3 and plain text ports are not to be used.",
+		"; These are optional and safe to leave out (e.g. if you have to click a lot in a",
+		"; DNS admin web interface).",
 		fmt.Sprintf(`_imap._tcp.%s.         IN SRV 0 1 143 .`, d),
 		fmt.Sprintf(`_submission._tcp.%s.   IN SRV 0 1 587 .`, d),
 		fmt.Sprintf(`_pop3._tcp.%s.         IN SRV 0 1 110 .`, d),
@@ -791,7 +793,7 @@ func IPs(ctx context.Context) ([]net.IP, error) {
 	}
 
 	// We'll list the IPs on the interfaces. How useful is this? There is a good chance
-	// we're listening on all addresses because of a load balancing/firewall.
+	// we're listening on all addresses because of a load balancer/firewall.
 	if ipv4all || ipv6all {
 		ifaces, err := net.Interfaces()
 		if err != nil {