pgsql: Arrange for function default arguments to be processed properly

Log Message:
-----------
Arrange for function default arguments to be processed properly in expressions
that are set up for execution with ExecPrepareExpr rather than going through
the full planner process.  By introducing an explicit notion of "expression
planning", this patch also lays a bit of groundwork for maybe someday
allowing sub-selects in standalone expressions.

Modified Files:
--------------
    pgsql/src/backend/commands:
        typecmds.c (r1.129 -> r1.130)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/typecmds.c?r1=1.129&r2=1.130)
    pgsql/src/backend/executor:
        README (r1.7 -> r1.8)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/README?r1=1.7&r2=1.8)
        execQual.c (r1.240 -> r1.241)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execQual.c?r1=1.240&r2=1.241)
    pgsql/src/backend/optimizer/plan:
        planner.c (r1.250 -> r1.251)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planner.c?r1=1.250&r2=1.251)
    pgsql/src/backend/optimizer/util:
        clauses.c (r1.274 -> r1.275)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/clauses.c?r1=1.274&r2=1.275)
        predtest.c (r1.23 -> r1.24)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/predtest.c?r1=1.23&r2=1.24)
    pgsql/src/include/optimizer:
        planner.h (r1.46 -> r1.47)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/planner.h?r1=1.46&r2=1.47)