Pa_SetHostApiInitializationMask to determine which host APIs will be initialized at startup

[rossb]

Change host API Ids to a bit mask and provide a function

Pa_SetHostApiInitializationMask( mask )

which can be called prior to Pa_Initialize() to determine which APIs will be initialized.

This could avoid initializing unwanted APIs (and all sorts of problems on Windows with buggy drivers when trying to init DirectSound? and ASIO and MME all for the same app.

[rossb]

Updated after marking #13 as a duplicate.

Note that making host API Ids a bit-mask rather than a sequential enumeration would limit us to 31 host apis in the future, but I sincerely hope we will never get to the point of supporting 31 host APIs anyway. Note that because some API functions return a HostAPI ID or a -ve error code, we can't use the high bit for an API id.

The default mask should be set so that if Pa_SetHostApiInitializationMask() isn't called, all available APIs are initialized.