Wintun and related improvements

1. Reference Wintun in the docs

2. Remove TAP-Windows from the installer

3. If -i not specified, we shouldn't continue to require TAP-Windows (see #239 (comment 568304674)). Instead, use vpninfo->hostname as Wintun adapter name, with TAP-Windows as a fallback only

4. On my Windows VM, I get permission errors with Wintun unless I run as Administrator.

   Is there some sudo-y thing that TAP-Windows is doing, and Wintun isn't? (Discussions with Wintun devs ongoing.) In the meantime, we should give the user an intelligible error.

5. Check for exit status of vpnc-script, even on Windows. (cf. vpnc-scripts!26 (merged))

6. Remove setting of Legacy IP address from within OpenConnect itself; leave this up to vpnc-script-win.js (working with the aforementioned branch from vpnc-scripts!26 (merged)).

7. add check_address_conflicts()

   Detect other adapters with conflicting IPv4/6 addresses, and try to use DeleteUnicastIpAddressEntry to delete-and-reclaim IP address from a down interface, so that vpnc-script-win.js will be able to set that address for the TAP/TUN.