[flang] Debugging of ACCESS='STREAM' I/O (take 2)

Corrects the runtime implementation of I/O on files with
the access mode ACCESS='STREAM'.  This is a collection
of edge-case tweaks to ensure that the distinctions between
stream and direct/sequential files, unformatted or formatted,
are respected where appropriate.

Moves NextInField() from io-stmt.h to io-stmt.cpp --
it was getting too big to keep in a header.

This patch exposed a problem with the I/O runtime
on Windows and it was reverted.  This version also
fixes that problem; files are now opened on Windows
in binary mode to prevent inadvertent insertions of
carriage returns before line feeds, and those line
endings (CR+LF) are now explicitly generated.

Differential Revision: https://reviews.llvm.org/D119015