read_until reworked so returned delimiter is explicit

[oli-obk]

read_line never returns '\n' character
read_until returns enum signifying whether delimiter is returned or not

not sure if this requires an rfc, the enum name is certainly bikesheddable.

[rust-highfive]

r? @pcwalton

(rust_highfive has picked a reviewer for you, use r? to override)

[alexcrichton]

Thanks for the PR! This is actively being redesigned as part of the I/O reform RFCs, however. Would you mind adding some comments to the relevant RFC instead?

We're currently going to largely freeze the current I/O system in order to prevent future breakage with it. We hope to take all known feedback into account with the upcoming I/O system (spelled out in RFCs), so please leave some feedback!