Detect running openvpn.exe processes that may prevent install/upgrade

The installer now checks if openvpn-gui.exe is running. If it is, the user must agree to close the GUI for the install to continue. By design, the GUI will close VPN connections when terminating, thereby leaving no openvpn.exe processes running. If an openvpn.exe is detected after the GUI has ended, the installer will not continue.

A new checkbox on the finish page "Start OpenVPN GUI" is automatically checked if the GUI was killed, it is left unchecked if not. The uninstaller will stop openvpn-gui.exe without prompting. 
String clean up.
Switch to NSIS MUI2 (vs MUI).