Make print, syswrite, send, readline, getc honour utf8-ness of PerlIO.

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.

p4raw-id: //depot/perlio@8054