Better Grant ownership metadata

Currently, grants track whether they are owned, allocator owned (a hack to avoid wasting initfs memory, and will be removed once the frame allocator supports expanding itself with new ranges), and an optional file descriptor they are associated with (for fmap). This is however insufficient, as a scheme with fmapped memory still in its address space, may funmap that memory while it is still available by the fmapper, and thus cause use-after-free.

Because schemes currently require root, this is not inherently worse than e.g. #82 (closed), but needs to be fixed soon. Fixing this would along with the addition of other kernel features (most notably scheme file forwarding), allow simplifying the proc scheme a lot, by making the kernel only provide access to memory by borrowing memory using fmap. It is also more or less necessary for on-demand paging.