add --directx flag

[SnowyWhite]

The launcher features a --dxvk flag to install DXVK for better AMD performance.
I think it would make sense to have an additional flag --directx to also install the DirectX End-User Runtime (June 2010).

The installation process would be quite simple:

• download the redistributable
• Create a temporary directory somewhere
• Run the executable silently to unpack itself to the temp folder: directx_Jun2010_redist.exe /Q /T:<temp_path>
• Run the extracted setup, also silently: ./<temp_path>/DXSETUP.exe /silent
• Remove the temporary directory and directx_Jun2010_redist.exe

The reason for this is that the runtime is required to run the game.
If a user tries to launch IW4x without running the game at least once via Steam first, the typical DirectX-related errors will occur (for example d3dx9_43.dll was not found).