Add nss-myhostname detection

Source /etc/rc.conf after /etc/conf.d/$service; do something intelligent
if the relevant variable is set in /etc/rc.conf (go to read-only mode?)

What happens if e.g. /etc/conf.d/hostname.{runlevel} files exist? Just
document that this case is not supported?

De-systemd-ize hwclock.c