Send the SIGALRM to the correct process when we timeout. See the note
in src/etc/rc regarding RC_PID.
The former code would leave you with a root shell when the prompt
timed out (if the tty is marked secure) while continuing the boot
process in another shell. This was easily hidden if you started
xdm(1) at boot, while the root shell remains on another tty
(accessible with CTL-ALT-F1).