1. null-safe interface to GiST

1. null-safe interface to GiST
   (as proposed in http://fts.postgresql.org/db/mw/msg.html?mid=1028327)

2. support for 'pass-by-value' arguments - to test this
   we used special opclass for int4 with values in range [0-2^15]
   More testing will be done after resolving problem with
   index_formtuple and implementation of B-tree using GiST

3. small patch to contrib modules (seg,cube,rtree_gist,intarray) -
   mark functions as 'isstrict' where needed.

Oleg Bartunov