Adopt BusyBox init signal handling for halt/reboot/poweroff

This patch adopts the BusyBox init signals for handling halt, reboot,
and poweroff.  The SIGINT and SIGPWR signals still behave the same as
before, even though they likely will become configurable in a future
release.

Note: the disabling of Ctrl-Alt-Delete (SIGINT) is also removed!

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

TODO.md

@@ -18,6 +18,17 @@ very low dependency on external packages.
 General
 -------
 
+* Add halt/poweroff/shutdown symlinks to finit and convert reboot, but
+  only install symlinks if these tools don't already exist.
+* Add reboot, halt, poweroff commands to `initctl` tool
+* SysV init and systemd use SIGUSR1 to restart their FIFO/D-Bus.  Add
+  API restart to SIGHUP callback for the same functionality in Finit.
+* Add `finit.conf` support for UPS notification (SIGPWR) to start a task
+  using, e.g. <sys/power/{ok,fail,low}> conditions.  More info in sig.c
+* Add `finit.conf` support for ctrl-alt-delete (SIGINT) and kbrequest,
+  i.e. KeyboardSignal, (SIGWINCH) behavior.  Using conditions to a task,
+  e.g, <sys/key/ctrlaltdel> and <sys/key/signal> like SIGPWR handling.
+* Add `IFF_RUNNING` support to netlink plugin along `IFF_IUP`
 * 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