Deparse.pm:pp_entersub: Refactor repetition away

I’m about to modify this code, and having two copies of the join(...)
is not so helpful.
Deparse & calls with scalar() where necessary

This is wrong:

$ ./perl -Ilib -mO=Deparse,-P -e 'sub foo($$){}; foo(bar(), baz())'
sub foo ($$) {

}
&foo(bar(), baz());
-e syntax OK

The resulting code calls bar and baz in list context.
deparse-skips.txt: comp/proto.t now passes
deparse-skips.txt: Note why comp/parser.t fails