refclock.cpp: Avoid a possible (but very unlikely) integer overflow.

Fixes CID 966081.
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
WebUI: Inform the web interface that no audio stream is available.
WebUI: Make use of new [hasaudio] info.
What the hell was intended here?!

Trying to make some sense of that code. Without much luck so far...
Options dialog: Disable "Reset" when the INI is read-only.

Ref #3591.