UID/GID identity and filesystems

Ideally, filesystems should have arbitrary UID/GID remapping capabilities: a translation layer that says what system UID/GID (in the current namespace) any given on-disk UID/GID maps to.

Ideally, that shouldn't require root intervention to set up; if you have permission to mount, you should be able to set up a map using any UIDs and GIDs you control. Supplementary GIDs give you the latter, and I have a supplementary UIDs patch that would provide the former.