Allow checking for duplicate bindings with -C

- Having both parse_configuration and parse_file is excessive now
- We detect if we are parsing only by checking if conn is NULL, not with
use_nagbar
- font.pattern needs to be set to NULL because it is freed in
free_font()

Fixes #3660

src/main.c

@@ -420,7 +420,7 @@ int main(int argc, char *argv[]) {
     }
 
     if (only_check_config) {
-        exit(parse_configuration(override_configpath, false) ? 0 : 1);
+        exit(load_configuration(override_configpath, C_VALIDATE) ? 0 : 1);
     }
 
     /* If the user passes more arguments, we act like i3-msg would: Just send
@@ -586,7 +586,7 @@ int main(int argc, char *argv[]) {
 #include "atoms.xmacro"
 #undef xmacro
 
-    load_configuration(override_configpath, false);
+    load_configuration(override_configpath, C_LOAD);
 
     if (config.ipc_socket_path == NULL) {
         /* Fall back to a file name in /tmp/ based on the PID */