[IMP] config: make log_handler and --log-handler additive

For log_handler (list of logger configuration specs), having the
configuration file and the CLI configuration be exclusive (one
overwriting the other) is detrimental: it precludes keeping the
configuration file as a convenient baseline and only altering the subset
of loggers of interest at any given time. Combine specs from both
sources instead of overwriting one with the other.

* remove log_handler and its special case from the first options loop
* remove seeding of option with DEFAULT_LOG_HANDLER
* my_default is the baseline "configuration file" value, it's None if
  not provided which is not convenient. Use DEFAULT_LOG_HANDLER for it
  as baseline configuration file. DEFAULT_LOG_HANDLER isn't a list
  anymore, it's a CSV of logger specs (same as file-serialized)
* could actually use a DEFAULT_LOG_HANDLER of ``:`` (the default logger
  is root, the default level is info), but that might be a tad too
  cryptic
* things are weird between my_default and the file-sourced value,
  _parse_config is first run with my_defaults then with the file, but if
  there's no file it's re-run with already-parsed my_defaults. So when
  the file and the command-line values are of different type,
  _parse_config must be ready to handle both
[IMP] config: deduplicate loggers in log_handler

Saving multiple levels for the same logger should work with few issues,
but over time pathological (basket) cases (e.g. using ``-s`` all the
time) may build pointlessly huge lists in their config file.

Only keep the last level for each logger when saving to a config file.