Add a _populate_part(ition) function that mimics the current
populate_slice. Note however, that this function is not
backward-compatible with populate_slice, hence the different name. A
"_" is prepended to signal that it still experimental.

It can be used to populate the /cfg and /data partition using makefs(8).

Initially not wired.

TODO:

• Related: https://github.com/freebsd/poudriere/issues/1181 With this approach, an mtree file will be required if a mix of different users/groups is desired.