Cash (http://pauillac.inria.fr/cash/) was an experiment in
using OCaml for shell scripting.  The project is no longer active and
the old code does not compile with OCaml 4.

Control flow can be simplified, removing the `do_syscall` variable.

To match `Val_emptylist`.

These I/O operations used to be defined as macros in <caml/io.h>, but
they are not speed-critical, so it's cleaner to have them as
functions, like all other I/O operations in this file.

Previously, output channels opened from C were also reported in this list,
causing `Stdlib.flush_all` to flush them.  This feels wrong, as
channels opened from C are entirely managed from C code.

With this commit, output channels opened from C always have a
reference count of 0: `caml_alloc_channel` is no longer called with
one of these channels, and it's the only function that increases refcount.
Hence the refcount check in `caml_close_channel` can be removed.

`caml_allc_custom_mem` cannot trigger finalization any longer.