pgsql: Simplify permissions for LOCK TABLE.

Simplify permissions for LOCK TABLE.

The prior behavior was confusing and hard to document. For instance,
if you had UPDATE privileges, you could lock a table in any lock mode
except ACCESS SHARE mode.

Now, if granted a privilege to lock at a given mode, one also has
privileges to lock at a less-conflicting mode. MAINTAIN, UPDATE,
DELETE, and TRUNCATE privileges allow any lock mode. INSERT privileges
allow ROW EXCLUSIVE (or below). SELECT privileges allow ACCESS SHARE.

Reviewed-by: Nathan Bossart
Discussion: https://postgr.es/m/9550c76535404a83156252b25a11babb4792ea1e.camel%40j-davis.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c44f6334ca6ff6d242d9eb6742441bc4e1294067

Modified Files
--------------
doc/src/sgml/ref/lock.sgml               | 25 ++++++------
src/backend/commands/lockcmds.c          | 18 ++++-----
src/test/regress/expected/privileges.out | 66 ++++++++++++++------------------
src/test/regress/sql/privileges.sql      | 61 ++++++++++++++---------------
4 files changed, 79 insertions(+), 91 deletions(-)