Fix a comment
Link to the correct sdl library for the loopback example
Add a method to set the running thread's name
Set a name for the mixer and recording threads
Add a cmake option to disable building only alsoft-config
Rework threading functions
Move SetThreadName to threads.c
Use an ALCbackend object to access playback backends

This is the start of a backend redesign. Currently, a wrapper object is used to
avoid having to redo all the backends at once, but they should slowly be
converted to derive from ALCbackend instead. The ALCbackend interface can
change as needed.
Move the ALCdevice handle to the ALCbackend base