filesink: Remove buffer, deprecate line-buffer mode and don't use fflush()

filesink: Remove buffer, deprecate line-buffer mode and don't use fflush()

fflush() has no effect because we use writev() directly, so fsync()
should be used instead which is actually flushing the kernel-side
buffers.

As a next step, a non-line-buffered buffering mode is to be added.

https://bugzilla.gnome.org/show_bug.cgi?id=794173