Raise an exception instead of Failure "execve: Argument list too long"

[Innf107]

Fix #556

[talex5]

This string matching looks a bit fragile (OS/locale dependent). Perhaps we should get action_execve to output errno and just match on that (e.g. execve:7:...)?

strerrorname_np is GNU-specific, but we can use caml_unix_error_of_code to convert integer codes to OCaml values.