ENH: improve OFstream append behaviour (#3160)

- previous support for file appending (unused in the meantime)
  specified opening with `std::ios_base::app`. However, this also
  enforces append behaviour for each write operation and thus
  disallows any seek/repositioning within the output file.

  Now treat append as an "append-like" behaviour instead.

  If the file already exists, its contents will be preserved and
  the *initial* output position is moved to the file end. All
  subsequent write operations respect the current file position
  without an additional seek-to-end on each write. This enables
  support of file appending/overwriting, but does preclude concurrent
  file output.