httpd: allow websockets VNC connection on http port if run with -enablehttpproxy

[maxnet]

Upstream noVNC defaults to making the VNC websockets connection on the http port instead of port 5900 and it would be nice if that would just work.

If run with -enablehttpproxy allow http connection to be upgraded to websockets. To test (on Debian or its derivates):

sudo apt install novnc
sudo ln -sf vnc_lite.html /usr/share/novnc/index.vnc
sudo x11vnc -httpport 80 -enablehttpproxy -httpdir /usr/share/novnc -no6 -xkb -repeat -auth guess -display WAIT:0 -forever -shared

Go with web browser to IP, and it should work without having to patch novnc or change settings.

[bk138]

A lot of changes to the guts, this will need a lot of testing for regressions.