Deprecate x11/libXp, it is deprecated upstream

Deprecate x11/libXp, since it is about to be deprecated upstream. Do *not*
yet set a removal date for x11/libXp, this will be decided later, once most
dependencies are fixed.
Deprecate all ports depending on x11/libXp.

• astro/xephem
• audio/xmcd
• audio/xmmix
• biology/ncbi-toolkit
• deskutils/xmdiary
• ftp/llnlxdir
• ftp/llnlxftp
• graphics/SciPlot
• misc/xtar
• multimedia/mpeg2play
• science/afni
• textproc/mgdiff

Sprinkle USES=xorg as needed.

Discussed with: bapt