Integrate perlio:

[  9039]
Missed added file.

[  9037]
Long-standing "defined but not used" SvPVbyte*() are about to get used
for auto-downgrade hooks in gethostbyaddr() etc. (as per Camel-III).
Make SvPVbyte*() symmetric with respect to SvPVutf8*() in that:
- it does a sv_utf8_downgrade (c.f. SvPVutf8's sv_utf8_upgrade())
- uses SvPV() rather than sv_2pv() in case it isn't a string.

p4raw-link: @9039 on //depot/perlio: 41010ee291c2b5acc799d2426bfb74f41a33ba21
p4raw-link: @9037 on //depot/perlio: 0875d2fe359ba14e879483eafe95813b77c5da09

p4raw-id: //depot/perl@9040