reboot: signal init, add -f option to call reboot();

reboot: signal init, add -f option to call reboot();
init: use SIGINT, use xsignal()

Now that oneit supports the same signals as our "sysv"-init for shutdown,
make reboot et al. signal pid 1 with the appropriate signal.
Of these signals, only SIGINT works with sysvinit 2.88, causing a reboot.
The others are only supported in Busybox init.

Also, make init accept SIGINT and use xsignal().