Stop recursion from losing lex fh names

Stop recursion from losing lex fh names

sub r {
    r($_[0]-1) if $_[0];
    open my $fh, "/dev/null";
    print "$_[0] $$fh\n"
}
r(5);
__END__

Output:
0 *main::
1 *main::
2 *main::
3 *main::
4 *main::
5 *main::$fh

The largest number represents the outermost call.

The handle name was being allocated as a target (a scratch variable
used by various operators to return values).  Targets are not shared
between recursion levels.

This commit tells pad_alloc to treat it like a constant, so it
is shared.