This is an efficiency change, and means we now no longer have to run
"out $_TD; local $_TD = shift;", which was especially pointless in the case of
non-trigger functions where the passed value was always undef anyway.

A tiny open issue is whether we should get rid of the $prolog argument of
mkfunc, and the corresponding pushed value, which is now just a constant "false".

Tim Bunce, reviewed by Alex Hunsaker.