An implementation of runtime host API selection (see ticket #10). See the changes to portaudio.h for documentation for the API. Note that unlike other proposals, this version of the interface allows the client to specify the order of host API initialization. The API is excercised by patest_select_hostapis.c. To reviewers: please verify that the implementation of Pa_SelectHostApis, Pa_GetSelectedHostApis and Pa_GetAvailableHostApis in pa_front.c matches the documentation in portaudio.h