Integrate from perlio:

[ 18591]
PERL_IMPLICIT_SYS does NOT vector PerlIO_xxx but PerlSIO_xxx, so
correct XSUB.h

[ 18611]
Michael Schroeder's fix for re-try if stdio ops after
interrupts. (Calls to PERL_ASYNC_CHECK added by NI-S).

[ 18612]
PerlIO friendly version of speed up of $/ = undef case.

[ 18613]
fflush() before invalidating stdio's fileno.
p4raw-link: @18613 on //depot/perlio: bbfd922f5bb0c97d0accf18f588b887418c94e6b
p4raw-link: @18612 on //depot/perlio: e468d35bbed7491e8b221d65181da6200264b6a0
p4raw-link: @18611 on //depot/perlio: 4d94824190f1d3f4307c94794486b277afd6e971
p4raw-link: @18591 on //depot/perlio: f9415d2377705b4cde579db69c15a29f1b9bb9f6

p4raw-id: //depot/perl@18616