pgsql: Mark a query's topmost Paths parallel-unsafe if they will have i

Mark a query's topmost Paths parallel-unsafe if they will have initPlans.

Andreas Seltenreich found another case where we were being too optimistic
about allowing a plan to be considered parallelizable despite it containing
initPlans. It seems like the real issue here is that if we know we are
going to tack initPlans onto the topmost Plan node for a subquery, we
had better mark that subquery's result Paths as not-parallel-safe. That
fixes this problem and allows reversion of a kluge (added in commit
7b67a0a49 and extended in f24cf960d) to not trust the parallel_safe flag
at top level.

Discussion: <874m2w4k5d(dot)fsf(at)ex(dot)ansel(dot)ydns(dot)eu>

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/474de765a8003bc58f5736f626bf533147cc1e68

Modified Files
--------------
src/backend/optimizer/plan/planner.c | 15 ++++++---------
src/backend/optimizer/plan/subselect.c | 9 ++++++---
2 files changed, 12 insertions(+), 12 deletions(-)