admin: in self-check for spf records against our ip's, don't try checking the unspecified addresses (0.0.0.0 and ::), and warn if there are no explicitly configured ips

based on question by spectral369 on #mox on matrix

mox-/admin.go

@@ -684,6 +684,9 @@ func DomainSPFIPs() (ips []net.IP) {
 		}
 		for _, ipstr := range ipstrs {
 			ip := net.ParseIP(ipstr)
+			if ip.IsUnspecified() {
+				continue
+			}
 			ips = append(ips, ip)
 		}
 	}