Integrate perlio:

[  8055]
Make write() (i.e. formats) utf8-aware by calling do_print() rather
than raw PerlIO_write() - does not seem to break any tests.

[  8054]
Make print, syswrite, send, readline, getc honour utf8-ness of PerlIO.
(sysread, recv and write i.e. formats still to do...)
Allow :utf8 or :bytes in PerlIO_apply_layers() so that
open($fh,">:utf8","name")
etc. work. - "applying" those just sets/clears the UTF8 bit of the top layer,
so no extra overhead is involved.
Tweak t/comp/require.t to add a 'use bytes' to permit its dubious writing of BOM
to a non-utf8 stream.
Add initial io/utf8.t
Fix SvPVutf8() - sv_2pv() was not expecting to be called with something
that was already SvPOK() - (we just fossiked with SvUTF8 bit). Fix that
and also just use the SvPV macro in sv_2pvutf8() to avoid the issue/overhead.

[  8053]
Did not get that has_utf8/this_utf8 fix right last time, another spot
was only testing this_utf8.

[  8052]
Typo/thinko in S_scan_const() - seeing high bit sets has_utf8 not this_utf8
i.e. the output string has one, but don't mess with source assumption.

p4raw-link: @8055 on //depot/perlio: d75029d0f4549b2fe0cff3cd80934a1b6bbb54eb
p4raw-link: @8054 on //depot/perlio: 7d59b7e40bca518078f3e97c802950b76d52efa2
p4raw-link: @8053 on //depot/perlio: bbc28b27949817e8e7461c0a92c6108632259a4b
p4raw-link: @8052 on //depot/perlio: 4e553d734e1cb450bb69a562e02eb0c12ecd8995

p4raw-id: //depot/perl@8058

diff --cc perlio.c
Simple merge

diff --cc pp_sys.c
Simple merge

diff --cc sv.c
Simple merge