pgsql: Teach the planner to support index access methods that only

Log Message:
-----------
Teach the planner to support index access methods that only implement
amgettuple or only implement amgetbitmap, instead of the former assumption
that every AM supports both APIs.  Extracted with minor editorialization
from Teodor's fast-GIN-insert patch; whatever becomes of that, this seems
like a simple and reasonable generalization of the index AM interface spec.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        catalogs.sgml (r2.198 -> r2.199)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/catalogs.sgml?r1=2.198&r2=2.199)
        indexam.sgml (r2.28 -> r2.29)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/indexam.sgml?r1=2.28&r2=2.29)
    pgsql/src/backend/optimizer/path:
        indxpath.c (r1.236 -> r1.237)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/indxpath.c?r1=1.236&r2=1.237)
    pgsql/src/backend/optimizer/util:
        plancat.c (r1.155 -> r1.156)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/plancat.c?r1=1.155&r2=1.156)
    pgsql/src/include/catalog:
        pg_am.h (r1.60 -> r1.61)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_am.h?r1=1.60&r2=1.61)
    pgsql/src/include/nodes:
        relation.h (r1.169 -> r1.170)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/relation.h?r1=1.169&r2=1.170)