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>

networking/udhcp/dhcpc.h

@@ -21,6 +21,9 @@ struct client_config_t {
 	uint8_t *vendorclass;           /* Optional vendor class-id to use */
 	uint8_t *hostname;              /* Optional hostname to use */
 	uint8_t *fqdn;                  /* Optional fully qualified domain name to use */
+
+	uint16_t first_secs;
+	uint16_t last_secs;
 } FIX_ALIASING;
 
 /* server_config sits in 1st half of bb_common_bufsiz1 */