Resource sharing between multiple NROs

[roblabla]

This is heavily related to #58 : We'll need to figure out a way to share resources between multiple started NROs, off the top of my head : filesystem hierarchy and open file descriptors. This would allow us to port stuff like bash without too much trouble, and it would "generalize" the way to pass file descriptors from the parent to the child (I'm thinking of stdin/stdout/stderr from ace_loader to the started process).

I think the best would be to have the libtransistor_context contain an optional pointer to the file descriptor array (or NULL if we don't want to pass any fds), which the child would copy into its own array. Something similar would probably be done to share a filesystem hierarchy.

[misson20000]

If we do that, then we should have a way for the loader to set the NRO's initial current directory, too, so it can mount its SquashFS root there.