fileio: tweak write_string_stream_ts() to write out trailing \n in one go even if...

fileio: tweak write_string_stream_ts() to write out trailing \n in one go even if buffering is off

This tweaks write_string_stream_ts() in one minor way: when stdio
buffering has been turned off, let's append the newline we shall append
to the buffer we write ourselves so that the kernel only gets one
syscall for the result. When buffering is enabled stdio will take care
of that anyway.

Follow-up for #7750.