Add --dhcp-split-relay option.

This makes a DHCPv4 relay which is functional when
client and server networks aren't mutually route-able.

src/dnsmasq.h

@@ -1148,6 +1148,7 @@ struct dhcp_relay {
   char *interface; /* Allowable interface for replies from server, and dest for IPv6 multicast */
   int iface_index; /* working - interface in which requests arrived, for return */
   int port;        /* Port of relay we forward to. */
+  int split_mode;  /* Split address allocation and relay address. */
 #ifdef HAVE_SCRIPT
   struct snoop_record {
     struct in6_addr client, prefix;
@@ -1666,6 +1667,8 @@ size_t dhcp_reply(struct dhcp_context *context, char *iface_name, int int_index,
 		  time_t recvtime, struct in_addr leasequery_source);
 unsigned char *extended_hwaddr(int hwtype, int hwlen, unsigned char *hwaddr, 
 			       int clid_len, unsigned char *clid, int *len_out);
+void relay_upstream4(int iface_index, struct dhcp_packet *mess, size_t sz, int unicast);
+struct dhcp_relay *relay_reply4(struct dhcp_packet *mess, char *arrival_interface);
 #endif
 
 /* dnsmasq.c */