Cleanup use of 16 that should be NAMEDATALEN.

src/backend/access/gist/gist.c

src/backend/access/heap/heapam.c

src/backend/storage/ipc/shmqueue.c

src/backend/storage/smgr/md.c

src/backend/tcop/dest.c

src/backend/utils/adt/regproc.c

src/backend/utils/adt/sets.c

src/backend/utils/cache/catcache.c

src/backend/utils/cache/syscache.c

src/include/libpq/libpq.h

src/include/storage/buf_internals.h

src/interfaces/libpq/fe-auth.c

src/interfaces/odbc/connection.h

src/test/regress/regress.c