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>

sig.h

@@ -66,6 +66,8 @@ typedef enum {
 	SHUT_REBOOT
 } shutop_t;
 
+extern shutop_t halt;
+
 void do_shutdown    (shutop_t op);
 int  sig_stopped    (void);
 void sig_init       (void);