Integrate:

Integrate:
[ 19286]
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.

and regen Configure.
p4raw-link: @19286 on //depot/perl: 15b61c98f82f3010e6eaa852f9fa5251de9e6dd9

p4raw-id: //depot/maint-5.8/perl@19295
p4raw-edited: from //depot/perl@19294 'edit in' config_h.SH (@19221..)
	Configure (@19278..)
p4raw-integrated: from //depot/perl@19294 'copy in' t/io/layers.t
	(@19243..) perlio.c (@19263..)