C++ inclusion fixes.

MSVC C++ has ssize_t and size_t types already. There is no need to define them

sometimes compilers/libraries contain max/min macros (for C++)
this may caase unclear error messages.

ignore trailing whitespace in bson_json_reader_read

This reverts commit 875199b4.

This reverts commit 70154643.

uses _open and _read on windows, open and read on posix.