When setting USE_XORG via OPTION helper, use comma syntax universally; in
`devel/sdl12', `graphics/blender', and `multimedia/libav' ports they were
set incorrectly, which caused build failures of `devel/sdl12' without the
OpenGL, for example.
Also, while at it, in `multimedia/libmpeg2', fix sed(1) based patching to
produce more neat results, and use official spelling of MPEG abbreviation
in COMMENT (port description still needs cleaning up).