Replace some appendStringInfo* calls with more appropriate variants

contrib/postgres_fdw/postgres_fdw.c

contrib/test_decoding/test_decoding.c

src/backend/access/rmgrdesc/smgrdesc.c

src/backend/access/rmgrdesc/xlogdesc.c

src/backend/catalog/namespace.c

src/backend/utils/adt/ruleutils.c