deps
handle stdin in Go land

turns out iomux doesn't actually do anything about stdin... wip.
deps
actually implement stdin streaming; replace iomux

iomux-spawn/link has been replaced with the much simpler and much easier to
test iodaemon. rather than muxing, which we never actually used, it simply
transmits the file descriptors for the spawned process to whoever requests
them. if two links occur, it will be arbitrarily split between them, and
that's OK (because we only ever have one link).