pgsql: Improve the IndexVacuumInfo/IndexBulkDeleteResult API to allow

Log Message:
-----------
Improve the IndexVacuumInfo/IndexBulkDeleteResult API to allow somewhat sane
behavior in cases where we don't know the heap tuple count accurately; in
particular partial vacuum, but this also makes the API a bit more useful
for ANALYZE. This patch adds "estimated_count" flags to both structs so
that an approximate count can be flagged as such, and adjusts the logic
so that approximate counts are not used for updating pg_class.reltuples.

This fixes my previous complaint that VACUUM was putting ridiculous values
into pg_class.reltuples for indexes. The actual impact of that bug is
limited, because the planner only pays attention to reltuples for an index
if the index is partial; which probably explains why beta testers hadn't
noticed a degradation in plan quality from it. But it needs to be fixed.

The whole thing is a bit messy and should be redesigned in future, because
reltuples now has the potential to drift quite far away from reality when
a long period elapses with no non-partial vacuums. But this is as good as
it's going to get for 8.4.

Modified Files:
--------------
pgsql/src/backend/access/gin:
ginvacuum.c (r1.28 -> r1.29)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginvacuum.c?r1=1.28&r2=1.29)
pgsql/src/backend/access/gist:
gistvacuum.c (r1.43 -> r1.44)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistvacuum.c?r1=1.43&r2=1.44)
pgsql/src/backend/access/hash:
hash.c (r1.110 -> r1.111)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hash.c?r1=1.110&r2=1.111)
pgsql/src/backend/access/nbtree:
nbtree.c (r1.169 -> r1.170)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtree.c?r1=1.169&r2=1.170)
pgsql/src/backend/catalog:
index.c (r1.316 -> r1.317)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/index.c?r1=1.316&r2=1.317)
pgsql/src/backend/commands:
analyze.c (r1.137 -> r1.138)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/analyze.c?r1=1.137&r2=1.138)
vacuum.c (r1.387 -> r1.388)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c?r1=1.387&r2=1.388)
vacuumlazy.c (r1.119 -> r1.120)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c?r1=1.119&r2=1.120)
pgsql/src/backend/postmaster:
pgstat.c (r1.187 -> r1.188)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c?r1=1.187&r2=1.188)
pgsql/src/include/access:
genam.h (r1.76 -> r1.77)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/genam.h?r1=1.76&r2=1.77)