Refactoring of channel output, media playback, and logging code.

I don't like making batches of unrelated changes like this, but
there was a bit of scope creep as I worke don some of this, and
I didn't want to commit a half-broken implementation.

Channel Output:
- Add USB dongle support for Lynx Pixelnet USB dongle.
- Add USB dongle support for DMXpro compatible devices.  This should
  support Entec Pro, Lynx DMX, DIYC RPM, and DMXking.com DMX dongles.
- Move channel output code into channeloutput directory and create
  a common channel output interface.
- Rename lightthread.* to channeloutput/channeloutputthread.*
- Removed PixelnetStubs.c in favor of adding in #define 'stubs' for
  the WiringPi functions in the main FPD code.  This allows us to
  compile and test everything in the main FPD code except the SPI code
  on non-Pi platforms.  With the channel output interface, the FPD
  code will not get executed unless the FPD is configured.

Media Playback:
- Move music playback code into mediaoutput directory and create
  a common media output interface.
- Add back MP3 audio file playback capability.  This uses mpg123 on
  the backend and allows uploading .mp3 files to the Audio directory
  via the File Manager screen.
- Add synchronized video file playback capability via omxplayer.
- Moved media/channel sync code into channeloutput/channeloutputthread.c
  so it can be used by the various media outputs.

Logging: (WORK IN PROGRESS, needs UI & to get rid of 'verbose' setting)
- Added concept of log level and facility to logging code.
  Default is Info level for Most facilities (minus channel data).
- Added in new LogInfo(), LogWarn(), and LogDebug() macros and VB_*
  #defines to facilitate logging at various levels and facilities
- Added "fpp -ll" command line option to set log level:
  "fpp -ll info"  "fpp -ll warn"   "fpp -ll debug"
- Added "fpp -lm" command line option to set log mask:
  "fpp -lm channel,mediaout,sequence"
  "fpp -lm all"
  "fpp -lm e131bridge,channelout"

Generic Changes/Enhancements:
- Add a quit command to fpp to signal fppd to quit gracefully so we
  don't have to kill the process.  'fpp -q'
- Removed double "-lm" in LIBS_fppd in Makefile