gui: Use lsu_merge_config_file_options().

gui: Use lsu_merge_config_file_options().

para_gui supports config file reloading, so parse_config_file_or_die()
needs to set MCF_DONT_FREE to keep a reference to the command line
parse result and MCF_OVERRIDE in the reload case. Also, the code to
check the key maps and to initialize the theme needs to be kept.