Add support for arbitrary prefix lengths in --rev-server and --domain=....,local

Previously, the prefix was limited to [8,16,24,32] for IPv4 and
to multiples of 4 for IPv6. This patch also makes the prefix-length optional
for --rev-server.

Inspired by a patch from DL6ER <dl6er@dl6er.de>, but completely
re-written by srk. All bugs are his.

src/network.c

@@ -1586,6 +1586,9 @@ void check_servers(int no_loop_check)
       if (serv->sfd)
 	serv->sfd->used = 1;
       
+      if (count == SERVERS_LOGGED)
+	my_syslog(LOG_INFO, _("more servers are defined but not logged"));
+      
       if (++count > SERVERS_LOGGED)
 	continue;