restore amd64 live image support for resize root after combined mbr/gpt commit

we need to resize_gpt now, as it takes precedence over mbr/disklabel
this change brings us to behave like the evbarm images.

XXX: we don't seem to touch disklabel and MBR, but they exist. Not sure whether
that has any negative repercussions, maybe another system might regard MBR as the
sole source of truth when GPT also exists.

/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: 1.2.2;
Check for right sysctl node to make live image work OOTB on OpenStack:

machdep.dmi.system-product = OpenStack Compute

amd64: liveimage: use RC_CONF_EXTRA instead of rc.conf.d