Support alternate database locations.

src/backend/parser/dbcommands.c

src/backend/storage/file/fd.c

src/backend/storage/smgr/md.c

src/backend/tcop/utility.c

src/backend/utils/init/postinit.c

src/include/miscadmin.h

src/include/nodes/parsenodes.h

src/include/parser/dbcommands.h