Cygwin: pty: Prevent garbled output for existing non-cygwin apps.

Cygwin: pty: Prevent garbled output for existing non-cygwin apps.

- If pseudo console is disabled, non-cygwin apps do not detect
  console device. In this case, some apps output UTF-8 regardless
  of the locale setting. At least git-for-windows, rust-based apps
  and node.js do that. This patch provides backward compatibility
  as default behaviour by setting console codepage to the charset of
  the locale. Even in the cases above, garbled output is prevented
  with this patch in most cases because mintty uses UTF-8 by default.

  I beleave this is not really a problem in cygwin side but that in
  app side, however, some users complain about garbled output with
  existing apps in MSYS2 (which is based on cygwin) in which pseudo
  console is disabled by default.