Add support for starting/stopping services on Netlink events

This commit adds support for event based services to Finit.  Along with
the previously added support for multiple instances of the same process,
the feature scope for Finit v2.0 has been reached!

As of now a service can be declared in /etc/finit.conf or /etc/finit.d/
like this:

    service :1 [2345] <!IFUP:eth0,GW> /sbin/dropbear -R -F -p 22 -- SSH daemon

Here the first instance `:1` of dropbear is declared to run in runlevels
2-5, but only if eth0 `IFUP:eth0` is up and a gateway `GW` is set.  When
the configuration changes, a new gateway is set, or somehow a new `IFUP`
event for eth0 is received, then dropbear is not SIGHUP'ed, but instead
stop-started `<!>`.  The latter trick applies to all services, even
those that do not define any events.

Currently inetd services are not supported for event based starting, but
it could easily be added.  Likely scenario is to deny incoming requests
on, e.g., eth0 if there is no gateway.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

plugins/netlink.c

@@ -18,7 +18,6 @@
 
 #include <errno.h>
 #include <net/if.h>		/* IFNAMSIZ */
-//#include <stdint.h>
 #include <sys/socket.h>
 #include <linux/types.h>
 #include <linux/netlink.h>
@@ -26,6 +25,7 @@
 #include <unistd.h>
 
 #include "../finit.h"
+#include "../event.h"
 #include "../helpers.h"
 #include "../plugin.h"
 
@@ -63,18 +63,18 @@ static void nl_route(struct nlmsghdr *nlmsg)
 		switch (a->rta_type) {
 		case RTA_GATEWAY:
 			gw = *((int *)data);
-			_d("GW: 0x%04x", gw);
+			//_d("GW: 0x%04x", gw);
 			break;
 
 		case RTA_DST:
 			dst = *((int *)data);
 			mask = r->rtm_dst_len;
-			_d("MASK: 0x%04x", mask);
+			//_d("MASK: 0x%04x", mask);
 			break;
 
 		case RTA_OIF:
 			idx = *((int *)data);
-			_d("IDX: 0x%04x", idx);
+			//_d("IDX: 0x%04x", idx);
 			break;
 		}
 
@@ -82,10 +82,13 @@ static void nl_route(struct nlmsghdr *nlmsg)
 	}
 
 	if ((!dst && !mask) && (gw || idx)) {
+		char msg[MAX_ARG_LEN];
+
 		if (nlmsg->nlmsg_type == RTM_DELROUTE)
-			_d("Default gateway 0x%4x removed", gw);
+			snprintf(msg, sizeof(msg), "GW:DN");
 		else
-			_d("New default gateway: 0x%4x", gw);
+			snprintf(msg, sizeof(msg), "GW:UP");
+		event_dispatch(msg);
 	}
 }
 
@@ -107,23 +110,28 @@ static void nl_link(struct nlmsghdr *nlmsg)
 
 	while (RTA_OK(a, la)) {
 		if (a->rta_type == IFLA_IFNAME) {
+			char msg[MAX_ARG_LEN];
+
 			strncpy(ifname, RTA_DATA(a), sizeof(ifname));
 			switch (nlmsg->nlmsg_type) {
 			case RTM_NEWLINK:
-				/* New interface has appearad, or interface flags has changed.
+				/*
+				 * New interface has appearad, or interface flags has changed.
 				 * Check ifi_flags here to see if the interface is UP/DOWN
 				 */
-				_d("New link (%s), UP: %d", ifname, i->ifi_flags & IFF_UP);
-				if (i->ifi_change & IFF_UP) {
-					int msg = (i->ifi_flags & IFF_UP) ? 1 : 0;
-
-					_d("%s: %s", ifname, msg ? "UP" : "DOWN");
-				}
+				if (i->ifi_change & IFF_UP)
+					snprintf(msg, sizeof(msg), "IF%s:%s",
+						 (i->ifi_flags & IFF_UP) ? "UP" : "DN",
+						 ifname);
+				else
+					snprintf(msg, sizeof(msg), "IFADD:%s", ifname);
+				event_dispatch(msg);
 				break;
 
 			case RTM_DELLINK:
 				/* NOTE: Interface has dissapeared, not link down ... */
-				_d("%s: DOWN", ifname);
+				snprintf(msg, sizeof(msg), "IFDEL:%s", ifname);
+				event_dispatch(msg);
 				break;
 
 			case RTM_NEWADDR: