Add in port to DG/UX

src/Makefile.global

src/backend/port/dgux/Makefile.inc

src/backend/port/dgux/dynloader.c

src/backend/port/dgux/machine.h

src/backend/port/dgux/port-protos.h

src/backend/port/dgux/port.c

src/mk/port/postgres.mk.dgux