pgsql: Allow GIN's extractQuery method to signal that nothing can

Log Message:
-----------
Allow GIN's extractQuery method to signal that nothing can satisfy the query.
In this case extractQuery should returns -1 as nentries. This changes
prototype of extractQuery method to use int32* instead of uint32* for
nentries argument.
Based on that gincostestimate may see two corner cases: nothing will be found
or seqscan should be used.

Per proposal at http://archives.postgresql.org/pgsql-hackers/2007-01/msg01581.php

PS tsearch_core patch should be sightly modified to support changes, but I'm
waiting a verdict about reviewing of tsearch_core patch.

Modified Files:
--------------
pgsql/contrib/intarray:
_int_gin.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/intarray/_int_gin.c.diff?r1=1.3&r2=1.4)
pgsql/contrib/tsearch2:
ginidx.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/tsearch2/ginidx.c.diff?r1=1.3&r2=1.4)
pgsql/doc/src/sgml:
gin.sgml (r2.7 -> r2.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/gin.sgml.diff?r1=2.7&r2=2.8)
pgsql/src/backend/access/gin:
ginarrayproc.c (r1.8 -> r1.9)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginarrayproc.c.diff?r1=1.8&r2=1.9)
ginbulk.c (r1.7 -> r1.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginbulk.c.diff?r1=1.7&r2=1.8)
ginget.c (r1.5 -> r1.6)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginget.c.diff?r1=1.5&r2=1.6)
gininsert.c (r1.6 -> r1.7)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/gininsert.c.diff?r1=1.6&r2=1.7)
ginscan.c (r1.8 -> r1.9)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginscan.c.diff?r1=1.8&r2=1.9)
ginutil.c (r1.9 -> r1.10)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginutil.c.diff?r1=1.9&r2=1.10)
pgsql/src/backend/utils/adt:
selfuncs.c (r1.223 -> r1.224)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.223&r2=1.224)
pgsql/src/include/access:
gin.h (r1.9 -> r1.10)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/gin.h.diff?r1=1.9&r2=1.10)