generate NSIS installers for Windows

Uses nsis to build Windows installers for OpenConnect (CLI only) as part of our CI's MingW32/64 cross-compilation.

The necessary nsiswrapper and makensis tools were added to the Fedora CI image in build-images@aa555f99

• Embeds the OpenVPN TAP driver (from https://build.openvpn.net/downloads/releases/tap-windows-9.21.2.exe)
  • TODO: #231 (closed), we should switch to the more modern wintun driver instead (Even OpenVPN itself has 😂)
• 64-bit builds now work (thanks to @dwmw2)
• Installs a .url shortcut file to the online documentation, rather than installing the HTML docs