Add WouldBlock error kind to IoError

[carllerche]

I'm working on a non-blocking IO lib for rust. It would be very helpful to have an error kind that clearly indicates the retry semantics of non-blocking IO.

I believe that currently EAGAIN and EWOULDBLOCK are mapped to ResourceUnavailable but that does not clearly indicate the would block / retry semantics.

[steveklabnik]

Adding stuff to the standard lib requires an RFC these days, if you still care about this, you should make sure the IO RFC takes care of it.

[steveklabnik]

/cc @aturon , i guess this is kinda a bug sorta

[steveklabnik]

nominating for backcompat-libs like all the othe rones

[carllerche]

/cc @alexcrichton

[aturon]

This currently maps to a ResourceUnavailable error (same as EINTR when it isn't retried internally).

[pnkfelix]

P-backcompat-libs, 1.0 beta, I-needs-decision