pgsql: Improve UPDATE/DELETE WHERE CURRENT OF so that they can be used

Log Message:
-----------
Improve UPDATE/DELETE WHERE CURRENT OF so that they can be used from plpgsql
with a plpgsql-defined cursor. The underlying mechanism for this is that the
main SQL engine will now take "WHERE CURRENT OF $n" where $n is a refcursor
parameter. Not sure if we should document that fact or consider it an
implementation detail. Per discussion with Pavel Stehule.

Modified Files:
--------------
pgsql/doc/src/sgml:
plpgsql.sgml (r1.111 -> r1.112)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.111&r2=1.112)
pgsql/src/backend/executor:
execCurrent.c (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execCurrent.c.diff?r1=1.1&r2=1.2)
execQual.c (r1.219 -> r1.220)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execQual.c.diff?r1=1.219&r2=1.220)
nodeTidscan.c (r1.54 -> r1.55)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeTidscan.c.diff?r1=1.54&r2=1.55)
pgsql/src/backend/nodes:
copyfuncs.c (r1.378 -> r1.379)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c.diff?r1=1.378&r2=1.379)
equalfuncs.c (r1.309 -> r1.310)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/equalfuncs.c.diff?r1=1.309&r2=1.310)
outfuncs.c (r1.310 -> r1.311)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/outfuncs.c.diff?r1=1.310&r2=1.311)
readfuncs.c (r1.208 -> r1.209)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/readfuncs.c.diff?r1=1.208&r2=1.209)
pgsql/src/backend/parser:
gram.y (r2.592 -> r2.593)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.592&r2=2.593)
parse_expr.c (r1.219 -> r1.220)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_expr.c.diff?r1=1.219&r2=1.220)
pgsql/src/backend/utils/adt:
ruleutils.c (r1.260 -> r1.261)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c.diff?r1=1.260&r2=1.261)
pgsql/src/include/executor:
executor.h (r1.140 -> r1.141)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/executor.h.diff?r1=1.140&r2=1.141)
pgsql/src/include/nodes:
primnodes.h (r1.131 -> r1.132)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/primnodes.h.diff?r1=1.131&r2=1.132)