Update GStreamer1 ports to 1.12.2.

Rework the adding of dependancies in Mk/bsd.gstreamer.mk.
Previous when using USE_GSTREAMER[1] it would just add the request modules to BUILD/RUN_DEPENDS. This caused the qa script to complain because the old code didn't implicit depend on the gstreamer1 and gstreamer1-plugins[-bad] ports for the libraries they carried, even if they where present via the plugins! The new code adds implicit depends on these ports so USE_GSTREAMER[1] using ports have all the libraries included.

• The mad mp3 plugin was removed, mpg123 plugin also provides mp3 decoding. Switch over ports that used the gstreamer1 mad plugin.
• gtksink plugin renamed -> gtk
• Hook up the sndio plugin into the framework
• Add some indirect dependacies where needed
• Reorder the plugin list in bsd.gstreamer.mk so only one plugin per line. When changing plugins it doesn't result in multiple lines being changed.
• Remove mentions in bsd.gstreamer.mk of plugins mentions that where removed.
• Depend on libunwind on i386/amd64, GStreamer links to it if it is present.

PR: 220753
Exp-run by: antoine@