ENH: improve OFstream append behaviour (#3160)

- previous support for file appending (largely unused) always
  specified opening with the std::ios_base::app flag.

  Now differentiate between append behaviours:

  APPEND_APP
  ~~~~~~~~~~
  Corresponds to std::ios_base::app behaviour:

  - Existing files will be preserved and a seek-to-end is performed at
    every write. With this mode seeks/repositioning within the file
    will effectively be ignored on output.

  APPEND_ATE
  ~~~~~~~~~~
  Largely approximates std::ios_base::ate behaviour:

  - Existing files will be preserved and a seek-to-end is performed
    immediately after opening, but not subsequently. Can use seekp()
    to overwrite parts of a file.