mlv_snd merged into mlv_rec and enabled by default

Changes discussed here: http://www.magiclantern.fm/forum/index.php?topic=12793

I did not test anything - I'm simply afraid to open the mlv_rec submenu. Seriously.

@stevefal, do you have a good proposal for cleaning up the menu?

For the private definitions, I would place them in the C source file (not just here, but everywhere they are used). If you ask me, the headers should contain only the public API. Other opinions?