Support for returning errno from generated stubs

[yallop]

This pull request adds support for returning errno alongside the return value of bound functions in generated stubs. As with #391, the interface changes are fairly modest: there is an additional argument, errno to write_c and write_ml, specifying the policy to use.

Besides the default, ignore_errno, there is one additional option, return_errno, which changes the return type of each function binding from t to t * int.

For example, here is the example bindings description for puts and rmdir from #391:

module Bindings(F: Cstubs.FOREIGN) =
struct
  open F
  let puts = foreign "puts" (string @-> returning int)
  let rmdir = foreign "rmdir" (string @-> returning int)
end

The C and OCaml portions of the bindings may be generated in the usual way, specifying return_errno with a keyword argument:

write_c fmt ~errno:return_errno ~prefix (module Bindings)
write_ml fmt ~errno:return_errno ~prefix (module Bindings)

In the module signature built by the linking step (i.e. by applying Bindings to the generated OCaml module) each function binding returns a pair of the return value of the bound C function and the value of errno just after the call:

(*
sig
  val puts : string -> int * int
  val rmdir : string -> int * int
end
*)
module C_errno = Bindings(Generated_module)

Errno and Lwt

There is also support for the combination of the new errno and concurrency options.

Passing ~concurrency:lwt_jobs to write_c and write_ml changes the return type of a function binding from t to Lwt.t (or rather to the wrapper type return).

Passing ~errno:return_errno changes the return type from t to t * int.

Passing both ~concurrency:lwt_jobs and ~errno:return_errno changes the return type from t to (t * int) Lwt.t --- that is, to an Lwt job that returns both the result of calling the bound C function and the value of errno just after the call.

[coveralls]

Coverage decreased (-0.5%) to 87.592% when pulling d883ab2 on yallop:errno-stubs into c6a99c1 on ocamllabs:master.