Add Pa_IsHostApiInstalled(hostApiTypeId) to detect installed host APIs

[PortAudio-admin]

Issue created by @RossBencina

The new Pa_GetAvailableHostApis() API (see ticket #10) is not sufficient to inform the application about which APIs will actually work, since it only returns the APIs which were compiled in to PortAudio. Host APIs that PortAudio dynamically loads (e.g. DirectSound, and ALSA (when certain build flags are set) may be returned by Pa_GetAvailableHostApis() but not actually present (so initializing them will yield no devices).

The current idea is to add a new function:

/** Probe for whether a host API is actually installed on the system.

@return Returns paNoError if the API is present, and an error otherwise, 
such as paInvalidHostApi if the host API is not compiled in, or 
paHostApiNotFound if the host API is not present on the system.
*/
PaError Pa_IsHostApiInstalled( PaHostApiTypeId hostApiTypeId );  

At the present time there are two APIs that could support this function: ALSA and DirectSound.

The implementation would be easiest by building off the ticket #10 work and adding an additional IsHostApiInstalled function pointer to each host APIs initializer entry.