Introduce two new Configure symbols:

Introduce two new Configure symbols:
[1] d_faststdio  = d_stdstdio && d_stdio_ptr_lval &&
                  (d_stdio_cnt_lval || d_stdio_ptr_lval_sets_cnt)
[2] usefaststdio = do we use fast stdio if we have it?
For 5.[68], we do.  For anything else, we don't.
(At least, unless otherwise instructed by -Dusefaststdio.)
This means that for bleadperl we no more use stdio, but instead
default to perlio: the effect of PERLIO=perlio, in other words.
(PERLIO=stdio will still switch to using stdio.)
This change may endanger extensions using FILE*-- but if we are
to migrate fully to perlio, better start swallowing the poison now.
For maintperl, the usefaststdio still defaults to yes.

p4raw-id: //depot/perl@19286