Stop writing down a pidfile.

Stop writing down a pidfile.

pid files aren't useful on OpenBSD, the rc.d(8) framework doesn't use
them.  In rtadvd, not writing a pid file means that we can set tighter
pledge(2) settings.  Also the pid file handling is pretty basic and the
atexit handler can't remove the pid file since rtadvd did a chroot - beurk.

ok florian@, "perfect" deraadt@ (!)