pgsql: Create new routines systable_beginscan_ordered,

Log Message:
-----------
Create new routines systable_beginscan_ordered, systable_getnext_ordered,
systable_endscan_ordered that have API similar to systable_beginscan etc
(in particular, the passed-in scankeys have heap not index attnums),
but guarantee ordered output, unlike the existing functions. For the moment
these are just very thin wrappers around index_beginscan/index_getnext/etc.
Someday they might need to get smarter; but for now this is just a code
refactoring exercise to reduce the number of direct callers of index_getnext,
in preparation for changing that function's API.

In passing, remove index_getnext_indexitem, which has been dead code for
quite some time, and will have even less use than that in the presence
of run-time-lossy indexes.

Modified Files:
--------------
pgsql/src/backend/access/heap:
tuptoaster.c (r1.85 -> r1.86)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/tuptoaster.c?r1=1.85&r2=1.86)
pgsql/src/backend/access/index:
genam.c (r1.66 -> r1.67)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/index/genam.c?r1=1.66&r2=1.67)
indexam.c (r1.105 -> r1.106)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/index/indexam.c?r1=1.105&r2=1.106)
pgsql/src/backend/catalog:
catalog.c (r1.74 -> r1.75)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/catalog.c?r1=1.74&r2=1.75)
pgsql/src/backend/storage/large_object:
inv_api.c (r1.131 -> r1.132)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/large_object/inv_api.c?r1=1.131&r2=1.132)
pgsql/src/backend/utils/cache:
ts_cache.c (r1.6 -> r1.7)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/ts_cache.c?r1=1.6&r2=1.7)
pgsql/src/include/access:
genam.h (r1.70 -> r1.71)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/genam.h?r1=1.70&r2=1.71)
relscan.h (r1.62 -> r1.63)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/relscan.h?r1=1.62&r2=1.63)
pgsql/src/include/catalog:
catalog.h (r1.38 -> r1.39)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catalog.h?r1=1.38&r2=1.39)