Add configurable log paths and address log path permission concerns

[jshort]

This is a feature request to allow an ini-config based and/or cxxopts way to change the location of the log files from /tmp to a directory of your choice:

• ideally the logfiles should go into a dedicated directory e.g. in
  /var/log/et for the server side.
• for the client side a set of logfiles can be kept in the home
  directory, but old logs should be deleted after some time.
  Alternatively client logs can be placed into /run/user//et-logs
  or a similar location.
• when using shared world-writable directories like /tmp then at least
  O_NOFOLLOW|O_EXCL|O_CREAT should also be added to open() flags. If
  open fails then it should be retried with alternative filenames until
  it succeeds.
• the file mode mask should be set in a way that the resulting files
  only have 0600 permissions i.e. read and write permission for the
  owner to avoid leaking information to other users on the system.