pgsql: Fix thinko in lock mode enum

Fix thinko in lock mode enum

Commit 0e5680f4737a9c6aa94aa9e77543e5de60411322 contained a thinko
mixing LOCKMODE with LockTupleMode. This caused misbehavior in the case
where a tuple is marked with a multixact with at most a FOR SHARE lock,
and another transaction tries to acquire a FOR NO KEY EXCLUSIVE lock;
this case should block but doesn't.

Include a new isolation tester spec file to explicitely try all the
tuple lock combinations; without the fix it shows the problem:

starting permutation: s1_begin s1_lcksvpt s1_tuplock2 s2_tuplock3 s1_commit
step s1_begin: BEGIN;
step s1_lcksvpt: SELECT * FROM multixact_conflict FOR KEY SHARE; SAVEPOINT foo;
a

1
step s1_tuplock2: SELECT * FROM multixact_conflict FOR SHARE;
a

1
step s2_tuplock3: SELECT * FROM multixact_conflict FOR NO KEY UPDATE;
a

1
step s1_commit: COMMIT;

With the fixed code, step s2_tuplock3 blocks until session 1 commits,
which is the correct behavior.

All other cases behave correctly.

Backpatch to 9.3, like the commit that introduced the problem.

Branch
------
REL9_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/51742063be4fd8d58fb8571e559fe8ca0abbbb57

Modified Files
--------------
src/backend/access/heap/heapam.c | 6 +-
src/test/isolation/expected/tuplelock-conflict.out | 469 ++++++++++++++++++++
src/test/isolation/isolation_schedule | 1 +
src/test/isolation/specs/tuplelock-conflict.spec | 63 +++
4 files changed, 537 insertions(+), 2 deletions(-)