plugin-scanner: rename plugin-scanner helper binary to gst-plugin-scanner

plugin-scanner: rename plugin-scanner helper binary to gst-plugin-scanner

and install into a different directory $(libexecdir/gstreamer-0.10) so that
everything is versioned properly.

NOTE: run 'make clean' after updating; if you are running an uninstalled setup,
you will need to update your gst-uninstalled script (unless it's symlinked
to gstreamer core master) and exit/enter your uninstalled environment to get
the updated environment. If you are running an installed setup, you should
run 'make uninstall' before merging this change or remove the old
plugin-scanner binary manually.

Fixes #601698.