pgsql: Code review for row security.

Code review for row security.

Buildfarm member tick identified an issue where the policies in the
relcache for a relation were were being replaced underneath a running
query, leading to segfaults while processing the policies to be added
to a query. Similar to how TupleDesc RuleLocks are handled, add in a
equalRSDesc() function to check if the policies have actually changed
and, if not, swap back the rsdesc field (using the original instead of
the temporairly built one; the whole structure is swapped and then
specific fields swapped back). This now passes a CLOBBER_CACHE_ALWAYS
for me and should resolve the buildfarm error.

In addition to addressing this, add a new chapter in Data Definition
under Privileges which explains row security and provides examples of
its usage, change \d to always list policies (even if row security is
disabled- but note that it is disabled, or enabled with no policies),
rework check_role_for_policy (it really didn't need the entire policy,
but it did need to be using has_privs_of_role()), and change the field
in pg_class to relrowsecurity from relhasrowsecurity, based on
Heikki's suggestion. Also from Heikki, only issue SET ROW_SECURITY in
pg_restore when talking to a 9.5+ server, list Bypass RLS in \du, and
document --enable-row-security options for pg_dump and pg_restore.

Lastly, fix a number of minor whitespace and typo issues from Heikki,
Dimitri, add a missing #include, per Peter E, fix a few minor
variable-assigned-but-not-used and resource leak issues from Coverity
and add tab completion for role attribute bypassrls as well.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/6550b901fe7c47c03775400e0c790c6c1234a017

Modified Files
--------------
doc/src/sgml/catalogs.sgml | 11 ++-
doc/src/sgml/config.sgml | 4 +-
doc/src/sgml/ddl.sgml | 168 ++++++++++++++++++++++++++++++++++
doc/src/sgml/ref/alter_table.sgml | 2 +-
doc/src/sgml/ref/create_policy.sgml | 2 +-
doc/src/sgml/ref/pg_dump.sgml | 17 ++++
doc/src/sgml/ref/pg_restore.sgml | 23 +++++
src/backend/catalog/heap.c | 2 +-
src/backend/catalog/system_views.sql | 2 +-
src/backend/commands/policy.c | 17 ++--
src/backend/commands/tablecmds.c | 4 +-
src/backend/rewrite/rowsecurity.c | 31 ++++---
src/backend/utils/adt/ri_triggers.c | 4 +-
src/backend/utils/cache/relcache.c | 91 +++++++++++++++++-
src/bin/pg_dump/pg_backup_archiver.c | 11 ++-
src/bin/pg_dump/pg_dump.c | 36 ++++----
src/bin/pg_dump/pg_dump.h | 2 +-
src/bin/pg_dump/pg_restore.c | 2 +-
src/bin/psql/describe.c | 79 +++++++++-------
src/bin/psql/tab-complete.c | 38 ++++----
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_class.h | 4 +-
src/include/commands/policy.h | 7 +-
src/test/regress/expected/rules.out | 2 +-
24 files changed, 439 insertions(+), 122 deletions(-)