pgsql: Implement an "S" option for psql's \dn command.

Implement an "S" option for psql's \dn command.

\dn without "S" now hides all pg_XXX schemas as well as information_schema.
Thus, in a bare database you'll only see "public". ("public" is considered
a user schema, not a system schema, mainly because it's droppable.)
Per discussion back in late September.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=e43fb604d6db229d70d3101aa53348cc16a5473a

Modified Files
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doc/src/sgml/ref/psql-ref.sgml | 9 +++++----
src/bin/psql/command.c | 2 +-
src/bin/psql/describe.c | 13 ++++++++-----
src/bin/psql/describe.h | 2 +-
4 files changed, 15 insertions(+), 11 deletions(-)