Disable unmaintained, fringe plugins by default

Submitted by Sebastian Dröge @slomo

Link to original bug (#774508)

Description

Considering https://scarybeastsecurity.blogspot.gr/2016/11/0day-exploit-compromising-linux-desktop.html and https://scarybeastsecurity.blogspot.gr/2016/11/0day-poc-risky-design-decisions-in.html, we should probably disable various plugins in gst-plugins-bad by default (and only enable directly or via --enable-unmaintained).

There are various others like these two plugins. I would consider everything that is basically unmaintained here, and plugins for very uncommon formats that have an alternative in ffmpeg (that works for us) or even in GStreamer.