Integrate perlio:

[ 16740]
#ifdef VMS code was not ported right from sv.c - s/fp/stdio/

[ 16739]
Fix for ungetc() issues flagged by ext/Encode/t/perlio.t on VMS.
ungetc() and buffer snooping may not mix. So use buffer snoop
hooks to avoid ungetc() where available.
unread() falls back to using :pending layer, and fill has
VMS specific code (which should not get used) or ungetc()
which should work.

[ 16737]
Flag the VMS-problem-causing part of :encoding
for enhancement. (Problem still needs fixing in perlio.c)
p4raw-link: @16740 on //depot/perlio: bad9695d2c27e100ff7ed9ad887696aa6f3d6b62
p4raw-link: @16739 on //depot/perlio: 936797854c70e5d5b5cc5ea02e2c3cbeffef5869
p4raw-link: @16737 on //depot/perlio: 8849edfd7e990957ceb72629c600ff2b74b838b1

p4raw-id: //depot/perl@16745