pgsql: Make UNKNOWN into an actual pseudo-type.

Make UNKNOWN into an actual pseudo-type.

Previously, type "unknown" was labeled as a base type in pg_type, which
perhaps had some sense to it because you were allowed to create tables with
unknown-type columns. But now that we don't allow that, it makes more
sense to label it a pseudo-type. This has the additional effects of
forbidding use of "unknown" as a domain base type, cast source or target
type, PL function argument or result type, or plpgsql local variable type;
all of which seem like good holes to plug.

Discussion: https://postgr.es/m/CAH2L28uwwbL9HUM-WR=hromW1Cvamkn7O-g8fPY2m=_7muJ0oA@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/d8d32d9a56a3cecfb14e8f47ebd50b780edffe60

Modified Files
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doc/src/sgml/datatype.sgml | 13 ++++++++++++-
doc/src/sgml/ddl.sgml | 2 +-
doc/src/sgml/plhandler.sgml | 2 +-
doc/src/sgml/protocol.sgml | 2 +-
doc/src/sgml/queries.sgml | 2 +-
doc/src/sgml/ref/create_function.sgml | 6 +++---
doc/src/sgml/ref/create_type.sgml | 2 +-
src/backend/catalog/heap.c | 3 +--
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_type.h | 2 +-
src/test/regress/expected/type_sanity.out | 5 ++---
src/test/regress/sql/type_sanity.sql | 2 +-
12 files changed, 26 insertions(+), 17 deletions(-)