ce1c570bfe
Provide much more detailed and structured discussion. Notably, try to gather
some of the conditions around the famous undefined behavior; these functions
(or the implementations) are quite prone to such behavior.
Add compatibility notes. (The big change here is that in SUSv4 it is no
longer mandated that the directory streams are implemented by using file
descriptors.)
XXX: Please proofread this. Consider also adding some additional notes. For
instance, I left the semantics about "." and ".." out. The returned
errno's should be enumerated as well.