Reenable DPI scaling in Windows 7

[mintty]

As reported under #352, when scaling the display to high DPI values in Windows 7, font rendering will result in blurred display.
The background is apparently that before mintty 2.2.1, the following section in the "manifest" (file res.mft) caused Windows to handle font DPI scaling smoothly:

  <asmv3:application xmlns:asmv3="urn:schemas-microsoft-com:asm.v3">
    <asmv3:windowsSettings xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">
      <dpiAware>true</dpiAware>
    </asmv3:windowsSettings>
  </asmv3:application>

When #470 added per-monitor DPI support for Windows 10 (release 2.2.1), this section had to be removed because the Windows API at that time would not work in coexistence with that configuration.
Later, #774 added DPI awareness mode V2. Unlike V1, this mode appears to work also with the above configuration, so it shall be reintroduced to mintty.

This enables proper DPI scaling again in Windows 7; the drawback is that proper DPI scaling will not work again in earlier Windows 10 versions (that do not support V2).

[mintty]

Released 3.0.2.