Integrate perlio:

[ 16211]
Re-instate $PerlIO::encoding::check at boot.
(Retaining Dan's XS side require though I don't see need.)

[ 16207]
Fix fd leak on Via(bogus).
Finish implementing PerlIOVia_open().
Export more guts of PerlIO_* so Via_open() can work.
Fix various PerlIO_allocate() features exposed by above.

[ 16027]
Fix perlio for Encode/t/perlio.t's SKIPPED TODO tests,
and change test not to skip them.

[ 16023]
Some tweaks for encodings which need lines.
Also disabled trial of using fallbacks.

[ 16022]
Various tweaks to Encode
p4raw-link: @16211 on //depot/perlio: 54871a3cda1a70e55971d42b5b2ac0aa06387aeb
p4raw-link: @16207 on //depot/perlio: d9dac8cda3a8128f09ab4445f683196e1392e4fa
p4raw-link: @16027 on //depot/perlio: e66821e8151d62edad66fb30ff4507c6d719f51b
p4raw-link: @16023 on //depot/perlio: 20de0cb2cf7a24aa48f2f9a26729f1a78e64f396
p4raw-link: @16022 on //depot/perlio: ca777f1ce1ff5cad164a769e07ab2e03b14695b6

p4raw-id: //depot/perl@16215