Fix buffer overflow of `pos` when file-size is too big

enable test

support IO#rewind

support IO#<<

Opened fd should be set FD_CLOEXEC by default

Made FILE_SEPERATOR platform dependent

define PATH_SEPARATOR

support for crossbuild for windows (32 bit and 64 bit) using mingw

Remove mrb_io_s_select typo.

Uncovered with a Coverity scan of our project using mruby-io. https://scan8.coverity.com/reports.htm#v26153/p11375/fileInstanceId=6844382&defectInstanceId=2515905&mergedDefectId=121921

Correct File::NULL for Windows

Excluding wait call for windows

IO.for_fd should call IO#initialize to initialize the created object
properly.  It cannot be implemented in Ruby.

FIX: IO#read create a large number of objects.

Revert "FIX: IO#read create a large number of objects."

FIX: IO#read create a large number of objects.

FIX: IO#sysread occurs SEGV

Fix broken build for Visual C++

Implement FileTest.size