PerlIO::scalar: stringify refs

If $s in
    open my $fh, "<",  \$s
or similar is a ref, then
stringify that ref: i.e. convert it from a ref into the string
"SCALAR(0x....)" or whatever.

This fixes

    RT #119529 Filehandle opened from ref to ref hangs on reading

which in this case was looping forever, since it kept thinking that
the var was a string ("SCALAR.."), but whose length was 0.

I haven't gone for a complete "always force var into a string" approach,
since PerlIO::scalar has quite a tolerance for "bad" vars; e.g.
it won't warn if $var is undef or a read-only constant number etc;
and it already normalises under some circumstances and not others.
So I've just increased the cases somewhat where it normalises.

Also, I didn't look to closely at the code that was looping (or to put it
another way, I looked at it but didn't understand it), so it could
conceivably still behave badly on some other strange type of variable that
manages to avoid getting normalised.