channels are never closed in Lisp Channel IO

[ivg]

The channel-close primitive just removes the binding but never closes so we eventually run out of available descriptors, especially, when we use the --run-in-isolation option where each job is creating its own set of channels.

Of course, care should be taken when we're closing a channel, so that we do not close our own standard channels, in case if they are not redirected, and propagate errors correctly to the return values (i.e., don't raise OCaml or Lisp exceptions in case of system errors).