pgsql: Make allow_system_table_mods settable at run time

Make allow_system_table_mods settable at run time

Make allow_system_table_mods settable at run time by superusers.  It
was previously postmaster start only.

We don't want to make system catalog DDL wide-open, but there are
occasionally useful things to do like setting reloptions or statistics
on a busy system table, and blocking those doesn't help anyone.  Also,
this enables the possibility of writing a test suite for this setting.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/8b00ea5e-28a7-88ba-e848-21528b632354%402ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c4a7a392ec8f0ff7701d84768080721ff8a7782e

Modified Files
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doc/src/sgml/config.sgml     | 8 +++++---
src/backend/utils/misc/guc.c | 2 +-
2 files changed, 6 insertions(+), 4 deletions(-)