pgsql: Allow new relmapper entries when allow_system_table_mods is true

Allow new relmapper entries when allow_system_table_mods is true.

This restores the pre-9.0 situation that it's possible to add new indexes
on pg_class and other mapped-but-not-shared catalogs, so long as you broke
the glass and flipped the big red Dont-Touch-Me switch. As before, there
are a lot of gotchas, and you'd have to be pretty desperate to try this
on a production database; but there doesn't seem to be a reason for
relmapper.c to be preventing such things all by itself. Per
experimentation with a case suggested by Cody Cutrer.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/f70f095c9096d5e2689e8d79172b37b57a84e51b

Modified Files
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src/backend/utils/cache/relmapper.c | 7 +++++--
1 files changed, 5 insertions(+), 2 deletions(-)