Opaque file objects

Problem

 * 1) See Virtualized privileged environment.
 * 2) In general, a program should not be unnecessarily aware of the non-capability environment around it, in particular absolute paths in the filesystem.
 * 3) We would like E to be compatible with actually interacting with a capability filesystem.

Furthermore, we want to accomplish this while still using java.io.File objects in E-on-Java.

Proposal
Currently,
 * the file__uriGetter is uniquely the root of the filesystem and has a distinct interface.
 * every File object reveals its absolute pathname.

Instead,
 * Every directory is equally usable as a root;
 * File objects cannot be asked for their pathnames; instead,  returns "/b/c" (or possibly "b/c"), much as a SturdyRef can only be converted to a cap-URL using.