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>

TODO.md

@@ -18,16 +18,10 @@ very low dependency on external packages.
 General
 -------
 
+* Add compile-time support for running `/bin/sh` instead of `getty`
 * Add `!` or `[wait <pidfile>]` to service directive to let finit pause
   and wait for a pidfile before starting processes -- both at boot and
   when starting processes in a runlevel.
-* Add `<GW,ADDR:iface,IF:iface>` events to service stanza so services
-  can be reloaded/restarted/stopped -- e.g, new GW --> SIGHUP process,
-  or loss of `IF:eth1` --> stop service, start it when eth1 comes up
-  again.
-* Add support for listening to netlink messages for GW, ADDR, IF UP/DOWN
-* Add `<!,...>` to let Finit know that a service does not support reload
-  (`SIGHUP`), but must be restarted (stop+start)
 * Allow custom reload for processes like libreSwan's pluto
 
         service [2345] <!gw,if:eth0,reload:'starter reload'> /sbin/pluto
@@ -38,7 +32,6 @@ General
   `.monitor=` in `svc_t` ... the script named as the basename of the
   service it monitors + `.sh`.
 * Add support for `/etc/sysctl.d`
-* Add compile-time supoprt for running `/bin/sh` instead of `getty`
 * Implement `initctl stop|start|restart|reload|status <SVC>` and
   `service <SVC> stop|start|restart|reload|status` on top
 * Add PRE and POST hooks for when switching between runlevels