More general code cleanup for readability.

More general code cleanup for readability.
A few more parameter validation checks.
Return DS_OK rather than 0 in a number of functions.
Fixed primary buffer ref count bug in CreateSoundBuffer().
Handle 1-3 byte sound fragments that would cause buffer overruns.
Clear primary buffer with a neutral value instead of always 0
(because 128 is neutral for 8-bit sound).
Fix bug with mixing 8-bit sound into the primary buffer.
Broke out the main block in DSOUND_thread() to another function for
readability.
Handle "no audio" and "audio busy" cases properly when initializing
dsound. Rename DllCanUnloadNow() to DSOUND_DllCanUnloadNow().