Add support for unique TFTP root per MAC.

It is currently only possible to let the TFTP server serve a different
folder depending on the client's IP address.
However it isn't always possible to predict what the client's
IP address will be, especially in situations in which we are not
responsible for handing them out (e.g. proxy dhcp setups).

Extend the current --tftp-unique-root parameter to support having a
separate folder per MAC address instead.

src/dnsmasq.h

@@ -211,7 +211,7 @@ struct event_desc {
 #define OPT_TFTP_SECURE    26
 #define OPT_TFTP_NOBLOCK   27
 #define OPT_LOG_OPTS       28
-#define OPT_TFTP_APREF     29
+#define OPT_TFTP_APREF_IP  29
 #define OPT_NO_OVERRIDE    30
 #define OPT_NO_REBIND      31
 #define OPT_ADD_MAC        32
@@ -238,7 +238,8 @@ struct event_desc {
 #define OPT_SCRIPT_ARP     53
 #define OPT_MAC_B64        54
 #define OPT_MAC_HEX        55
-#define OPT_LAST           56
+#define OPT_TFTP_APREF_MAC 56
+#define OPT_LAST           57
 
 /* extra flags for my_syslog, we use a couple of facilities since they are known 
    not to occupy the same bits as priorities, no matter how syslog.h is set up. */