Pull out the selection of ctypes_call/ctypes_call_errno.
Pull out the selection of ctypes_call/ctypes_call_errno.

dsheets/ocaml-ctypes,ocamllabs/ocaml-ctypes,ocamllabs/ocaml-ctypes,yallop/ocaml-ctypes,fdopen/ocaml-ctypes,sjfloat/ocaml-ctypes,yallop/ocaml-ctypes,fdopen/ocaml-ctypes,whitequark/ocaml-ctypes,sjfloat/ocaml-ctypes,whitequark/ocaml-ctypes,whitequark/ocaml-ctypes,dsheets/ocaml-ctypes,dsheets/ocaml-ctypes,sjfloat/ocaml-ctypes,fdopen/ocaml-ctypes