pgsql: Create a new type category for "internal use" types.

Create a new type category for "internal use" types.

Historically we've put type "char" into the S (String) typcategory,
although calling it a string is a stretch considering it can only
store one byte. (In our actual usage, it's more like an enum.)
This choice now seems wrong in view of the special heuristics
that parse_func.c and parse_coerce.c have for TYPCATEGORY_STRING:
it's not a great idea for "char" to have those preferential casting
behaviors.

Worse than that, recent patches inventing special-purpose types
like pg_node_tree have assigned typcategory S to those types,
meaning they also get preferential casting treatment that's designed
on the assumption that they can hold arbitrary text.

To fix, invent a new category TYPCATEGORY_INTERNAL for internal-use
types, and assign that to all these types. I used code 'Z' for
lack of a better idea ('I' was already taken).

This change breaks one query in psql/describe.c, which now needs to
explicitly cast a catalog "char" column to text before concatenating
it with an undecorated literal. Also, a test case in contrib/citext
now needs an explicit cast to convert citext to "char". Since the
point of this change is to not have "char" be a surprisingly-available
cast target, these breakages seem OK.

Per report from Ian Campbell.

Discussion: https://postgr.es/m/2216388.1638480141@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/07eee5a0dc642d26f44d65c4e6263304208e8583

Modified Files
--------------
contrib/citext/expected/citext.out | 7 ++++++-
contrib/citext/expected/citext_1.out | 7 ++++++-
contrib/citext/sql/citext.sql | 3 ++-
doc/src/sgml/catalogs.sgml | 4 ++++
src/bin/psql/describe.c | 4 ++--
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_type.dat | 14 +++++++-------
src/include/catalog/pg_type.h | 1 +
8 files changed, 29 insertions(+), 13 deletions(-)