[flang] Emit unformatted headers & footers even with RECL=

The runtime library was emitting unformatted record headers and
footers when an external unit had no fixed RECL=.  This is wrong
for sequential files, which should have headers & footers even
with RECL.  Change to omit headers & footers from unformatted
I/O only for direct access files.

Differential Revision: https://reviews.llvm.org/D112243