pgsql: Change patternsel (LIKE/regex selectivity estimation) so that if

Log Message:
-----------
Change patternsel (LIKE/regex selectivity estimation) so that if there
is a large enough histogram, it will use the number of matches in the
histogram to derive a selectivity estimate, rather than the admittedly
pretty bogus heuristics involving examining the pattern contents. I set
'large enough' at 100, but perhaps we should change that later. Also
apply the same technique in contrib/ltree's <@ and @> estimator. Per
discussion with Stefan Kaltenbrunner and Matteo Beccati.

Modified Files:
--------------
pgsql/contrib/ltree:
ltree_op.c (r1.12 -> r1.13)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/ltree/ltree_op.c.diff?r1=1.12&r2=1.13)
pgsql/src/backend/utils/adt:
selfuncs.c (r1.212 -> r1.213)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.212&r2=1.213)
pgsql/src/include/utils:
selfuncs.h (r1.34 -> r1.35)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/selfuncs.h.diff?r1=1.34&r2=1.35)