Fix #282: add support for oncrash:script to call post:script action

This patch adds the oncrash:script option to call the post:script
action, if defined, for a crashing service.  The EXIT_CODE variable
sent to the script is set to `crashed`.

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

doc/config.md

@@ -549,7 +549,8 @@ configurable with the following options:
     was originally designed for so not the default behavior
   * `oncrash:reboot` -- when all retries have failed, and the service
     has *crashed*, if this option is set the system is rebooted.
-    Note, future releases may include other `oncrash:` actions
+  * `oncrash:script` -- similarly, but instead of rebooting, call the
+    `post:script` action if set, see below.
 
 When stopping a service (run/task/sysv/service), either manually or
 when moving to another runlevel, Finit starts by sending `SIGTERM`, to