Initialize only adrivers based on actual needs

[derselbst]

As reported by @ReinholdH:

Buggy soundcard drivers have caused portaudio to segfault. If fluidsynth is compiled with support for portaudio it will always attempt to Pa_Initialize() within new_fluid_settings() (even if the user doesnt want to use portaudio) and thus causing fluidsynth to crash as well as any application using fluidsynth.

Thus introduce the ability to register only certain audio drivers based on acutal needs. Achieve this similar to other projects (like FFMpeg) by introducing

int fluid_register_adriver(...)

which can be called once after fluidsynths startup to register audio driver that shall be usable throughout fluidsynths lifetime. If this function is not being called, all available audio drivers are registered by default.

If possible, have this functionality for the next maintenance release, else for the next major one.