all pathname-on-server related definitions to MI code, since they are now
defined to be consistent across all platforms. Also, nuke the "rels" global,
since release numbers are now always specified in "long form", i.e. 1.3A
vs. 13A.
* Use md_make_bsd_partitions() code lifted from the i386 version,
modified to set D partition size.
* Add the MD messages used by the above to sysisnt/arch/pmax/msg.md.eng.
* Add X11 sets xbase, xserver, xcomp to arch/pmax/md.h.