privsep: Reduce fd use

privsep: Reduce fd use

On start close all FD's above stderr.
Close some fd's we don't need in processes spawned from priv.
Ensure we init some FD's to -1 to ensure we don't close stdin.
If DEBUG_FD is defined, we log FD's opened by pid.
Audit process FD usage and document it so I don't forget it.

Fixes #316.