Instead of adding a new artifact.h to pray.c, remove the existing
ones from attrib.c, invent.c, and mkobj.c. This also updates the Unix
and VMS editions of Makefile.src; having stale dependencies in those
for other ports could cause unnecessary recompilation but can't break
anything in this case.
Update the instructions, add a dat -> util dependency so that
MMK (freeware clone of DEC's MMS make utility) builds the data files
when necessary, and switch the default compiler to DEC C.
