pgsql: Put back allow_system_table_mods check in heap_create().

Put back allow_system_table_mods check in heap_create().

This reverts commit a475c6036752c26dca538632b68fd2cc592976b7.

Erik Rijkers reported back in January 2013 that after the patch, if you do
"pg_dump -t myschema.mytable" to dump a single table, and restore that in
a database where myschema does not exist, the table is silently created in
pg_catalog instead. That is because pg_dump uses
"SET search_path=myschema, pg_catalog" to set schema the table is created
in. While allow_system_table_mods is not a very elegant solution to this,
we can't leave it as it is, so for now, revert it back to the way it was
previously.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/15386281a62071f322155b74cb32c2fea92e8350

Modified Files
--------------
src/backend/bootstrap/bootparse.y | 3 ++-
src/backend/catalog/heap.c | 18 ++++++++++++++++--
src/backend/catalog/index.c | 3 ++-
src/include/catalog/heap.h | 3 ++-
4 files changed, 22 insertions(+), 5 deletions(-)