Import fdopen patch for windows

[hhugo]

This is a straight import of the patch used to fix ocamlbuild on windows inside opam-repository.

Opened as a Draft for now.

I've imported it on top of #329 so that we can have a CI.
I've added a commit to make the testsuite pass again on my machine.

[hhugo]

• Here is a list of changes present in this PR and not in master.

1. handle long command on windows (above 8191 bytes), by moving the command to a separate shell script.
2. use a leading space instead of '' for bash commands (it looks nicer)
3. handle Unix.EPIPE gracefully when Sys.win32 in ocamlbuild_executor
4. normalize path with forward slash
5. set stdout to binary mode to avoid CRLF

We might want to import 1 and 2, I don't know about 3. We can probably ignore 4 and 5

[hhugo]

3 was fixed in OCaml 4.05

• #7342, #797: fix Unix.read on pipes with no data left on Windows
  it previously raised an EPIPE error, it now returns 0 like other OSes
  (Jonathan Protzenko, review by Andreas Hauptmann and Damien Doligez)

[hhugo]

2 -> #343

[hhugo]

1. handle long command on windows (above 8191 bytes), by moving the command to a separate shell script.

@dra27, now that we no longer rely on cmd.exe do we still have such a low limit ? A quick search online says that createProcess has a limit of 32767.

[hhugo]

1. is implemented in Add a tests for long commands  #344