Try making PerlIO errors more consistent.

Try making PerlIO errors more consistent.
(1) Check early whether the filehandle is capable of
    the requested action.
(2) PerlIO_read() really can return -1, and so can
    PerlIOUnix_read().
(3) Some stdio implementations are more forgiving
    than others, so check also for errors recorded
    by PerlIO in PerlStdio_errno().

p4raw-id: //depot/perl@20050