Log message:
- Change libao to not build OSS, Sun, MacOS X nor several other plugins even
  when the header files are found.  Bump its revision to 1 due to this.
  This makes the libao package completely audio-output agnostic.
- Add a libao-macosx package containing the MacOS X plugin (sorry, untested).
- Add a libao-oss package containing the OSS plugin.  While here, use
  ossaudio.buildlink3.mk to determine the audio device instead of hardcoding
  it in a patch.
- Add a libao-macosx package containing the Sun audio plugin (sorry, untested).

Change approved by silence on tech-pkg@ for quite a long time since my
proposal.

I'm doing these changes to cleanup the base libao package.  Before these,
it contained a huge conditional to determine when the OSS and/or Sun
plugins were built, which often failed in untested platforms (PLIST
inconsistencies, for example).  Having these plugins in independent packages
makes debugging easier, keeps the dependency tree clean and also lets us use
{ONLY,NOT}_FOR_PLATFORM directives to easily say where they work or don't.