Rethinking splice()

- splice composes with tee/vmsplice/etc. to allow more complex zero-copy IO flows

- readthenwrite could not replace splice for this purpose, because splice has *super bizarre* semantics that are different from readthenwrite, as this article notes. (In particular, readthenwrite from a file into a pipe has to make a copy -- it skips bouncing through userspace, but it has to copy data from the page buffer into the pipe buffer, instead of doing wacky things with page pointers like splice does.) But those bizarre semantics are what let compositions of splice/tee/vmsplice/etc. be zero-copy.

I don't think it really works out; this stuff really needs to be replaced with something better. But I can at least see why splice seemed like a good idea at the time.