pgsql: Minor preparatory refactoring for UPDATE row movement.

Minor preparatory refactoring for UPDATE row movement.

Generalize is_partition_attr to has_partition_attrs and make it
accessible from outside tablecmds.c. Change map_partition_varattnos
to clarify that it can be used for mapping between any two relations
in a partitioning hierarchy, not just parent -> child.

Amit Khandekar, reviewed by Amit Langote, David Rowley, and me.
Some comment changes by me.

Discussion: http://postgr.es/m/CAJ3gD9fWfxgKC+PfJZF3hkgAcNOy-LpfPxVYitDEXKHjeieWQQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ef6087ee5fa84206dc24ba1339e229354b05cf2a

Modified Files
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src/backend/catalog/partition.c | 87 +++++++++++++++++++++++++++++++++++-----
src/backend/commands/tablecmds.c | 71 +++-----------------------------
src/include/catalog/partition.h | 6 ++-
3 files changed, 87 insertions(+), 77 deletions(-)