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

libi3/font.c

@@ -163,6 +163,7 @@ i3Font load_font(const char *pattern, const bool fallback) {
 
     i3Font font;
     font.type = FONT_TYPE_NONE;
+    font.pattern = NULL;
 
     /* No XCB connction, return early because we're just validating the
      * configuration file. */