RFC: fullfs — disk-full simulation filesystem

Hello tech-kern,

I've written the fullfs that simulates a full disk based on nullfs. 
The patch includes the kernel module, mount_full(8), 
rump library wiring, and ATF tests. I've tested against a netbsd vm.

I am not familiar with patches through email, so I've created a PR 
in Github: https://github.com/NetBSD/src/pull/76

While working on it I've noticed the nullfs_init only exists in the
header file. So I've created another PR removing it:
https://github.com/NetBSD/src/pull/75

Would appreciate any feedback.
Best Regards, Hudson