Create the pg_namespace system catalog. Doesn't do much yet, but it's

doc/src/sgml/catalogs.sgml

src/backend/catalog/Makefile

src/backend/catalog/indexing.c

src/backend/catalog/pg_namespace.c

src/backend/commands/command.c

src/backend/utils/cache/syscache.c

src/include/catalog/catalog.h

src/include/catalog/catname.h

src/include/catalog/catversion.h

src/include/catalog/indexing.h

src/include/catalog/pg_namespace.h

src/include/utils/syscache.h

src/test/regress/expected/sanity_check.out