vmimage.subr: Add missing directories to METALOG

[cperciva]

Summary

Theoretically METALOG should include everything which needs to go
into disk images; unfortunately there are still a few bugs which
are resulting in directories not being listed -- and if METALOG
has files in unrecorded directories, the directories end up being
created with 000 permissions.

Oddly enough, systems where / has 000 permissions are not very
usable.

As a temporary hack, compare the staging tree against METALOG and
add entries for any unrecorded directories. This will hopefully
be reverted before 15.0-RELEASE.

Sponsored by: https://www.patreon.com/cperciva
MFC after: 5 minutes

[cperciva]

I've tested this with releng/15.0 and it produces usable EC2 AMIs, so I'd like to insta-merge this and start 15.0-BETA2 builds with it about 9 hours from now.

[bapt]

should fix the issues spotted by scaleway people as well

[ivy]

i notice we don't put / in a package, but i also recall there's some reason this doesn't work. until we can fix that properly, this seems fine.