Initial work towards various IO cleanups with an eye to fixing https://github.com/fish-shell/fish-shell/issues/110
Reenable free_redirected_fds_from_pipes.
Trivial cleanup of a function in proc.cpp
Big fat refactoring of how redirections work. In fish 1.x and 2.0.0, the redirections for a process were flattened into a big list associated with the job, so there was no way to tell which redirections applied to each process. Each process therefore got all the redirections associated with the job. See  https://github.com/fish-shell/fish-shell/issues/877 for how this could manifest.

With this change, jobs only track their block-level redirections. Process level redirections are correctly associated with the process, and at exec time we stitch them together (block, pipe, and process redirects).

This fixes the weird issues where redirects bleed across pipelines (like #877), and also allows us to play with the order in which redirections are applied, since the final list is constructed right before it's needed.  This lets us put pipes after block level redirections but before process level redirections, so that a 2>&1-type redirection gets picked up after the pipe, i.e. it should fix https://github.com/fish-shell/fish-shell/issues/110

This is a significant change. The tests all pass. Cross your fingers.
Replace some #warnings with a comment explaining why the code is OK