Extract config file handlers into a separate file

We used custom_load_conf() to load the configuration file and
io_save_conf() to save configuration. Move these functions, including
all helpers, to a central location.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/dmon.c

@@ -163,7 +163,7 @@ dmon_start (int parent_exit_status)
   if (!io_file_exist (path_conf))
     DMON_ABRT (_("Could not access \"%s\": %s\n"),
                path_conf, strerror (errno));
-  custom_load_conf ();
+  config_load ();
 
   if (!io_file_exist (path_apts))
     DMON_ABRT (_("Could not access \"%s\": %s\n"),