nvmm: group

[Maxime Villard <max%m00nbsd.net@localhost>]

Currently /dev/nvmm has these permissions

        c rw- --- --- root wheel

Which means the user has to tune the permissions manually. I would like to
introduce an "nvmm" group, and change the perms to

        c rw- r-- --- root nvmm

This means that emulators will have to be setgid nvmm, or the caller must
be part of nvmm.

Also, I want to make nvmm_init() public. Until now it was invoked during
the first NVMM call within libnvmm. From now on it's the emulators that
must invoke it before any other NVMM function. The point is that the
emulator now has the ability to manually set the proper permissions before
invoking nvmm_init() and drop them after the call has completed.

Here is a patch [1] that adds this group (are there more changes needed?),
and makes nvmm_init public.

Feel free to comment, I'll commit it soon.

[1] https://m00nbsd.net/garbage/nvmm/nvmm-group.diff