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
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/82634a88d1ba2fdc3308d589b0b8f3bfdbd880f1

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