Deparse.pm: Remove special \0 marker

In 4b1385ee6 I did not realise we already had \cK, which served almost
the same purpose.
Deparse.pm: Remove temp pre-PADNAME code

added in d4f1bfe749f, which got merged before the PADNAME changes.
[perl #123357] Fix deparsing of $; at stmt end

Instead of sometimes appending ; to statements and then removing it
later, to avoid doubling it up, *only* append ; to a statement when
actually joining them together or emitting them.  That fixes bugs with
do{$;} becoming do {$} and ‘$_=$;; $;=7;’ becoming ‘$_=$; $;=7;’.

I also removed the boilerplate from pp_stub, since it was redundant
(and slow) and also partially wrong.  The $name var was bogus.