Allow plpgsql IN parameters to be assigned to. Since the parameters are just

Allow plpgsql IN parameters to be assigned to.  Since the parameters are just
preinitialized local variables, this does not affect the function's semantics
as seen by callers; allowing assignment simply avoids the need to create more
local variables in some cases.  In any case we were being rather inconsistent
since only scalar parameters were getting marked constant.

No documentation change, since parameters were never documented as being
marked constant anyway.

Steve Prentice