mixer: Use lsu_merge_config_file_options().

mixer: Use lsu_merge_config_file_options().

para_mixer is special in the way the config file is handled because
it is the only executable which allows options for subcommands
to be stored in the config file. Therefore the mixer specific
parse_and_merge_config_file() is called twice. First to merge the
global (supercommand) options, then to merge the subcommand options.
This does not cause any problems regarding the conversion, though.