ruby.c: use UTF-8 for argv

* ruby.c (process_options, ruby_script, ruby_set_argv): convert
  argv from UTF-8.

* win32/win32.c (rb_w32_sysinit, cmdglob, w32_cmdvector): convert
  wide char command line to UTF-8 argv.

* win32/win32.c (win32_direct_conv, rb_w32_readdir): convert UTF-8
  by using Win32 API directly.