Assorted message style improvements

src/backend/access/spgist/spgutils.c

src/backend/access/transam/xlog.c

src/backend/utils/adt/rangetypes.c

src/backend/utils/misc/guc.c

src/backend/utils/time/snapmgr.c

src/bin/pg_basebackup/streamutil.c

src/bin/pg_ctl/pg_ctl.c

src/bin/psql/common.c

src/bin/psql/variables.c

src/interfaces/libpq/fe-connect.c

src/interfaces/libpq/win32.c

src/pl/plpgsql/src/gram.y

src/test/regress/expected/plpgsql.out