pgsql: Replace simple constant pg_am.amcanreturn with an AM support fun

Replace simple constant pg_am.amcanreturn with an AM support function.

The need for this was debated when we put in the index-only-scan feature,
but at the time we had no near-term expectation of having AMs that could
support such scans for only some indexes; so we kept it simple.  However,
the SP-GiST AM forces the issue, so let's fix it.

This patch only installs the new API; no behavior actually changes.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/3695a555136a6d179cac8ae48d5f90171d5b30e9

Modified Files
--------------
doc/src/sgml/catalogs.sgml            |   15 +++++----
doc/src/sgml/indexam.sgml             |   24 ++++++++++-----
src/backend/access/index/indexam.c    |   22 ++++++++++++++
src/backend/access/nbtree/nbtree.c    |   11 +++++++
src/backend/access/spgist/spgscan.c   |    7 ++++
src/backend/optimizer/path/indxpath.c |    4 +-
src/backend/optimizer/util/plancat.c  |    2 +-
src/include/access/genam.h            |    1 +
src/include/access/nbtree.h           |    1 +
src/include/access/spgist.h           |    1 +
src/include/catalog/catversion.h      |    2 +-
src/include/catalog/pg_am.h           |   52 ++++++++++++++++----------------
src/include/catalog/pg_proc.h         |    4 ++
src/include/nodes/relation.h          |    2 +-
src/include/utils/rel.h               |    1 +
15 files changed, 103 insertions(+), 46 deletions(-)