Change to runlevel 0/6 on reboot/halt/poweroff

This patch fixes a long-standing issue with how Finit handles shutdown,
reboot and poweroff -- and even runlevel changes to runlevel 0 and 6.

Prior to this patch sig.c:do_shutdown() was called immediately on any
of the above events, disregarding any services that a user might have
wanted to run as part of the shutdown/reboot.

With this change the handling is unified in all frontends to instead
do a runlevel change followed by the do_shutdown() action.

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

TODO.md

@@ -20,8 +20,6 @@ General
 
 * Add halt/poweroff/shutdown symlinks to finit and convert reboot, but
   only install symlinks if these tools don't already exist.
-* Step state machine at shutdown instead of killing, same as shifting to
-  runlevel 0 or 6, then call external (newly installed?) tool to reboot.
 * Add support for timed shutdown in Finit, including cancelled shutdown.
 * SysV init and systemd use SIGUSR1 to restart their FIFO/D-Bus.  Add
   API restart to SIGHUP callback for the same functionality in Finit.