Teach mdmfs about tmpfs and use tmpfs in rc scripts

[Jia-Shiun Li]

On Sun, Mar 9, 2014 at 1:42 AM, Ian Lepore <ian at freebsd.org> wrote:
>
> Part of the reason I jumped on this when you brought it up on the arm
> list is because we're heavy users of the readonly rootfs support rc
> scripts at $work (and not many people are), so I wanted to make sure the
> change wasn't done on the assumption that you can just hard-code tmpfs
> in place of md in the rc scripts "and stuff will just keep working".
> That's only true for some limited subset of "stuff" and while it's
> certainly possible to manually change your rc.conf to match when
> upgrading, that's actually hard to do when you're trying to manage
> software ranging from freebsd 6 through 11 all with a common config and
> build and deployment system.
>

Does FreeBSD have policy or guideline on how to maintain
compatibility? After all, consider the tmpfs case it will be literally
neither md nor mfs.

That said, guess that is too trivial to bikeshed. Functionally I agree
it is good. It maintains compatibility while making use of better tmpfs.
Do you plan to commit it?

-Jia-Shiun.