/etc/rc.d/cerctl_init: New script for certctl rehash in live images.

This is very limited -- it does not supplant postinstall to rehash
certificates on upgrade; it only runs certctl rehash if
/etc/openssl/certs is an empty directory, as you get in live images
not created with sysinst.

We could also have a more general-purpose way to run postinstall(8)
on first boot of an image, but that has a lot more moving parts to
think about, so let's start with this limited-scope low-risk
approach.

PR install/57629

XXX pullup-10

branches: 1.1.2;
evbarm: liveimage: use RC_CONF_EXTRA instead of rc.conf.d