socket peeking function

[daurnimator]

From discussion back on 6th January (filing as issue now as I wanted to something public to refer to):

@wahern: it occurred to me that a good generic read interface would be pread. pread would be like a peek which accepted all the formats xread does, except it also could take an offset. and pread doesn't dequeue the data; that would be done manually (or through a special flag to pread).
@daurnimator: pread is already a posix-y function; bad name
@wahern: pread would also make it easier to build a peg-like parser around the interface.
@wahern: but the semantics would be almost identical.
@daurnimator: could have it as a flag to xread?
@daurnimator: precendent: recv() has MSG_PEEK.
@wahern: it's really messy to add to xread. a flag that signals another optional argument is even messier than it already is.
@wahern: but w'ever it's called, it could effectively replace the low-level recv call as the basic building block.
@daurnimator: sounds plausible
@daurnimator: would be good to see a prototype I guess
@wahern: it had it almost completely written at one point. in my head. the critical thing is that it makes speculative reads easy. no reading and putting back, or 0 timeouts, etc.
@daurnimator: it would also need to return the offset in the read buffer of where it was found
@daurnimator: (so that if you want to e.g. read a line followed by a length, you know where to start the length from)
@wahern: um. you provide the offset. but it would need to provide the trailing offset, so that the caller can discard and/or skip over the data when the returned string isn't the same length as the source string in the buffer (i.e. because of EOL translation).
@wahern: yeah. similar to what you said.