qmmp: Block out older FFMpeg supports

Drop OPTIONs for FFMPEG_LEGACY, FFMPEG1, and FFMPEGDEVEL though
they are still supported by upstream.  FFMpeg port core maintainer
wg@ said multimedia/ffmpeg0 for FFMPEG_LEGACY will be removed soon,
and multimedia/ffmpeg1 for FFMPEG1 and multimedia/ffmpeg-devel for
FFMPEGDEVEL were already moved away from ports tree.
libbs2b: Use pathfix macro to correct pkgconfig path
Use the new options framework

opusfile, qmmp, qmmp-plugin-pack:
  ${option_knob}_LIB_DEPENDS= ...
  ${option_knob}_BUILD_DEPENDS= ...
  ${option_knob}_USE= ...
  ${option_knob}_CONFIGURE_ENABLE= ...

Use additional macro multimedia/qmmp/options.qmake.mk to simplify
qmake directives with immitating Mk/bsd.options.mk style:
  ${option_knob}_QMAKE_CONFIG= ...
  ${option_knob}_QMAKE_CXXFLAGS= ...

opusfile, qmmp:
  Replace option knob APIDOC with DOXYGEN that comes from general
  option of Mk/bsd.options.desc.mk .
qmmp:
  Replace option knob PULSE_AUDIO with PULSEAUDIO that comes from
  general option of Mk/bsd.options.desc.mk .