Fix a segfault in handle_interface: if we're adding an interface that
we already knew about (in particular due to a udev event), then
calling init_state/start_interface on ifp is bad because we later free
ifp. This leads to messages like:

  dhcpcd[1342]: eth0: IAID conflicts with one assigned to eth0

and then a crash:

  Invalid read of size 8
    at 0x41CD07: dhcp_handlepacket (dhcp.c:2555)
    by 0x408A74: eloop_start (eloop.c:399)
    by 0x4073B6: main (dhcpcd.c:1537)

  Address 0x54573d8 is 200 bytes inside a block of size 232 free'd
    at 0x4C27507: free (in /nix/store/jafbjc2y6izmh3sk78fl65n55jll4mj8-valgrind-3.9.0/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    by 0x40F0A8: free_interface (net.c:153)
    by 0x405443: handle_interface (dhcpcd.c:764) by 0x57E0F06: ??? (udev.c:97)
    by 0x42EB62: dev_handle_data (dev.c:153)
    by 0x408A74: eloop_start (eloop.c:399)
    by 0x4073B6: main (dhcpcd.c:1537)

So ignore interfaces that we already know about.

diff -ru -x '*~' dhcpcd-6.4.0-orig/dhcpcd.c dhcpcd-6.4.0/dhcpcd.c
--- dhcpcd-6.4.0-orig/dhcpcd.c	2014-06-14 22:13:12.000000000 +0200
+++ dhcpcd-6.4.0/dhcpcd.c	2014-07-03 11:13:39.133186533 +0200
@@ -774,11 +774,11 @@
 		} else {
 			TAILQ_REMOVE(ifs, ifp, next);
 			TAILQ_INSERT_TAIL(ctx->ifaces, ifp, next);
-		}
-		if (action > 0) {
-			init_state(ifp, ctx->argc, ctx->argv);
-			run_preinit(ifp);
-			dhcpcd_startinterface(ifp);
+			if (action > 0) {
+				init_state(ifp, ctx->argc, ctx->argv);
+				run_preinit(ifp);
+				dhcpcd_startinterface(ifp);
+			}
 		}
 	}