Misc changes to channel output, media playback, and logging code.

- Fix debug log output from channel output thread
- Fix DMXPro dongle type in usbdongle.php
- Remove DMXPro outputData array since we can write directly from input
  data
- Fix DMXPro max channels to 512 instead of 4096
- (re)Disable USBPixelnet output in bridge mode until it's fully
  tested
- Rework serial output setup code to be a bit more flexible
- Add DMXOpen support (needs testing with a FTDI dongle, this is not
  compatible with CH341 dongles)