pgsql: Change patternsel() so that instead of switching from a pure

Log Message:
-----------
Change patternsel() so that instead of switching from a pure
pattern-examination heuristic method to purely histogram-driven selectivity at
histogram size 100, we compute both estimates and use a weighted average.
The weight put on the heuristic estimate decreases linearly with histogram
size, dropping to zero for 100 or more histogram entries.
Likewise in ltreeparentsel(). After a patch by Greg Stark, though I
reorganized the logic a bit to give the caller of histogram_selectivity()
more control.

Modified Files:
--------------
pgsql/contrib/ltree:
ltree_op.c (r1.16 -> r1.17)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/ltree/ltree_op.c?r1=1.16&r2=1.17)
pgsql/src/backend/utils/adt:
selfuncs.c (r1.244 -> r1.245)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c?r1=1.244&r2=1.245)
pgsql/src/include/utils:
selfuncs.h (r1.43 -> r1.44)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/selfuncs.h?r1=1.43&r2=1.44)