Handle HTTP for threaded server

[bk138]

If you'd like to put out an incentive for implementing this feature, you can do so at https://issuehunt.io/r/LibVNC/libvncserver

Is your feature request related to a problem? Please describe.

The embedded HTTP server code is not executed at all when the library runs in threaded mode.

Describe the solution you'd like

Working HTTP server when the library runs in threaded mode.

Describe alternatives you've considered

None.

Additional context

https://github.com/LibVNC/libvncserver/blob/master/libvncserver/main.c#L623