Allow user namespaces on provided. linux runtimes

[Cloudef]

Currently it's not possible to create an user namespace and give the process its own view of the filesystem hierarchy. It would be great if this could be allowed to allow easy bundling of nix derivations into AWS lambdas.

The currently failing operation is writing to /proc/self/uid_map
https://github.com/Cloudef/zig2nix/blob/master/src/bundle/loader/src/namespace.zig#L141

[joshtriplett]

I'm interested in this as well, both for its own sake and in order to get permissions to mount additional filesystems within the user namespace.