pgsql: Make wal_compression PGC_SUSET rather than PGC_USERSET.

Make wal_compression PGC_SUSET rather than PGC_USERSET.

When enabling wal_compression, there is a risk to leak data similarly to
the BREACH and CRIME attacks on SSL where the compression ratio of
a full page image gives a hint of what is the existing data of this page.
This vulnerability is quite cumbersome to exploit in practice, but doable.

So this patch makes wal_compression PGC_SUSET in order to prevent
non-superusers from enabling it and exploiting the vulnerability while
DBA thinks the risk very seriously and disables it in postgresql.conf.

Back-patch to 9.5 where wal_compression was introduced.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/19a65458159ca5f46d8ac154e62273fa2a8cf13f

Modified Files
--------------
doc/src/sgml/config.sgml | 1 +
src/backend/utils/misc/guc.c | 2 +-
2 files changed, 2 insertions(+), 1 deletion(-)