pgsql: Disallow extensions from owning the schema they are assigned to.

Disallow extensions from owning the schema they are assigned to.

This situation creates a dependency loop that confuses pg_dump and probably
other things. Moreover, since the mental model is that the extension
"contains" schemas it owns, but "is contained in" its extschema (even
though neither is strictly true), having both true at once is confusing for
people too. So prevent the situation from being set up.

Reported and patched by Thom Brown. Back-patch to 9.1 where extensions
were added.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/4d642b59417d5d4a62d126ca12a80da45d2a3fbf

Modified Files
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src/backend/commands/extension.c | 30 +++++++++++++++++++++++++++---
1 files changed, 27 insertions(+), 3 deletions(-)