Bunch of reworking to how we manage audio devices.

Device enumeration now happens at startup and then is managed exclusively
through hotplugging instead of full redetection. The device name list now has
a unique "handle" associated with each item and SDL will pass this to the
backend so they don't have to figure out how a human readable name maps to
real hardware for a second time.

Other cleanups, fixes, improvements, plus all the audio backends updated to
the new interface...largely untested at this point, though.

src/audio/SDL_audiodev_c.h

@@ -33,7 +33,6 @@
 #define OPEN_FLAGS_INPUT (O_RDONLY|O_NONBLOCK)
 #endif
 
-void SDL_EnumUnixAudioDevices(int iscapture, int classic,
-                              int (*test) (int fd), SDL_AddAudioDevice addfn);
+extern void SDL_EnumUnixAudioDevices(const int classic, int (*test)(int));
 
 /* vi: set ts=4 sw=4 expandtab: */