Add support for sys/pwr/fail and sys/key/ctrlaltdel conditions

This patch drops the default signal handlers for SIGPWR and SIGINT with
optional tasks in /etc/finit.conf, triggered by two new conditions:
sys/pwr/fail and sys/key/ctrlaltdel, respectively.

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

doc/conditions.md

@@ -115,6 +115,8 @@ Built-in conditions:
 - `net/<IFNAME>/up`
 - `net/<IFNAME>/running`
 - `sys/pwr/ac`
+- `sys/pwr/fail`
+- `sys/key/ctrlaltdel`
 - `usr/foo`
 
 **Note:** `up` means administratively up, the interface flag `IFF_UP`.