pgsql: Rename 'cmd' to 'cmd_name' in CreatePolicyStmt

Rename 'cmd' to 'cmd_name' in CreatePolicyStmt

To avoid confusion, rename CreatePolicyStmt's 'cmd' to 'cmd_name',
parse_policy_command's 'cmd' to 'polcmd', and AlterPolicy's 'cmd_datum'
to 'polcmd_datum', per discussion with Noah and as a follow-up to his
correction of copynodes/equalnodes handling of the CreatePolicyStmt
'cmd' field.

Back-patch to 9.5 where the CreatePolicyStmt was introduced, as we
are still only in alpha.

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/49f9a2831b8c5c8941eec9baa5d44b471971704e

Modified Files
--------------
src/backend/commands/policy.c  |   22 +++++++++++-----------
src/backend/nodes/copyfuncs.c  |    2 +-
src/backend/nodes/equalfuncs.c |    2 +-
src/backend/parser/gram.y      |    2 +-
src/include/nodes/parsenodes.h |    2 +-
5 files changed, 15 insertions(+), 15 deletions(-)