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).
Add "KEYWORD: interactive" so that the script's prompts work,
and use rc_print_metadata to add a message to the log.
Part of the /etc/rc silent changes requested in PR 41946
and proposed in tech-userlevel.
not absolute.
This affects an update with sysinst, which will try to look at
/targetroot/etc/fstab, which points to /etc/etc.whatever, which is
not there during the upgrade, and results in all (/targetroot)/etc
files getting nuked!
