udhcpc: emit "correct" secs field

In theory, sending secs set to constant zero should be ok too.
But some bleeping servers can actually be configured to answer ONLY
if secs is bigger than a preset value (!!)
http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man8/bootpd.8.html
grep for "reply_threshold_seconds"

function                                             old     new   delta
udhcpc_main                                         2573    2623     +50

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/networking/udhcp/dhcpc.c b/networking/udhcp/dhcpc.c
index e9a8412..ca82d37 100644
--- a/networking/udhcp/dhcpc.c
+++ b/networking/udhcp/dhcpc.c
@@ -407,11 +407,19 @@
 /* Initialize the packet with the proper defaults */
 static void init_packet(struct dhcp_packet *packet, char type)
 {
+	uint16_t secs;
+
 	/* Fill in: op, htype, hlen, cookie fields; message type option: */
 	udhcp_init_header(packet, type);
 
 	packet->xid = random_xid();
 
+	client_config.last_secs = monotonic_sec();
+	if (client_config.first_secs == 0)
+		client_config.first_secs = client_config.last_secs;
+	secs = client_config.last_secs - client_config.first_secs;
+	packet->secs = htons(secs);
+
 	memcpy(packet->chaddr, client_config.client_mac, 6);
 	if (client_config.clientid)
 		udhcp_add_binary_option(packet, client_config.clientid);
@@ -848,6 +856,7 @@
 	/* else LISTEN_NONE: sockfd stays closed */
 }
 
+/* Called only on SIGUSR1 */
 static void perform_renew(void)
 {
 	bb_info_msg("Performing a DHCP renew");
@@ -1260,6 +1269,7 @@
 			case BOUND:
 				/* 1/2 lease passed, enter renewing state */
 				state = RENEWING;
+				client_config.first_secs = 0; /* make secs field count from 0 */
 				change_listen_mode(LISTEN_KERNEL);
 				log1("Entering renew state");
 				/* fall right through */
@@ -1299,6 +1309,7 @@
 				bb_info_msg("Lease lost, entering init state");
 				udhcp_run_script(NULL, "deconfig");
 				state = INIT_SELECTING;
+				client_config.first_secs = 0; /* make secs field count from 0 */
 				/*timeout = 0; - already is */
 				packet_num = 0;
 				continue;
@@ -1315,6 +1326,7 @@
 		/* note: udhcp_sp_read checks FD_ISSET before reading */
 		switch (udhcp_sp_read(&rfds)) {
 		case SIGUSR1:
+			client_config.first_secs = 0; /* make secs field count from 0 */
 			perform_renew();
 			if (state == RENEW_REQUESTED)
 				goto case_RENEW_REQUESTED;
@@ -1446,6 +1458,7 @@
 							udhcp_run_script(NULL, "deconfig");
 						change_listen_mode(LISTEN_RAW);
 						state = INIT_SELECTING;
+						client_config.first_secs = 0; /* make secs field count from 0 */
 						requested_ip = 0;
 						timeout = tryagain_timeout;
 						packet_num = 0;
@@ -1493,6 +1506,7 @@
 				change_listen_mode(LISTEN_RAW);
 				sleep(3); /* avoid excessive network traffic */
 				state = INIT_SELECTING;
+				client_config.first_secs = 0; /* make secs field count from 0 */
 				requested_ip = 0;
 				timeout = 0;
 				packet_num = 0;