pgsql: Tweak fillfactor code as per my recent proposal.

Log Message:
-----------
Tweak fillfactor code as per my recent proposal. Fix nbtsort.c so that
it can handle small fillfactors for ordinary-sized index entries without
failing on large ones; fix nbtinsert.c to distinguish leaf and nonleaf
pages; change the minimum fillfactor to 10% for all index types.

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
create_index.sgml (r1.54 -> r1.55)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_index.sgml.diff?r1=1.54&r2=1.55)
pgsql/src/backend/access/gin:
ginutil.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginutil.c.diff?r1=1.3&r2=1.4)
pgsql/src/backend/access/nbtree:
nbtinsert.c (r1.139 -> r1.140)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtinsert.c.diff?r1=1.139&r2=1.140)
nbtsort.c (r1.104 -> r1.105)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtsort.c.diff?r1=1.104&r2=1.105)
pgsql/src/include/access:
gist_private.h (r1.21 -> r1.22)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/gist_private.h.diff?r1=1.21&r2=1.22)
hash.h (r1.71 -> r1.72)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/hash.h.diff?r1=1.71&r2=1.72)
nbtree.h (r1.100 -> r1.101)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/nbtree.h.diff?r1=1.100&r2=1.101)