FileOutputStream_md.c should be merged into FileOutputStream.c

Once bug

6631352: File{OutputStream,Writer} should implement atomic append mode using FILE_APPEND_DATA (win)

is fixed, the windows and unix versions of FileOutputStream_md.c
will be almost identical. They can be merged into one platform-independent file,
for maintainability.

Other refactorings are possible, e.g.
passing the file descriptor/HANDLE explicitly as a jlong instead of accessing
implicitly via JNI reflection.