fix: accept keycode-only modifiers in parse_mod - #1388

The keys docs show binds that use keycodes as the modifier, like bind=code:64,code:24,killclient and bind=code:64+code:133,code:24,killclient, but neither of them loads. The config check prints Unknown modifier: code:64 and the bind is dropped.

In parse_mod(), the code: branch ORs the right modifier bit into mod but never sets match_success, so when every token is a keycode the function falls through to the !match_success path and returns UINT32_MAX. A mixed form like SUPER+code:50 only works because the named modifier sets the flag. This started with bcee63f (the config check added in 0.12.0), and since parse_mod() is shared it also affects mousebind, axisbind, gesturebind and the global keybinding window rule.

The fix sets match_success in the four recognised keycode cases, the same way the named-modifier branch does. An unknown keycode such as code:24 still ends up as an unknown modifier.

I checked it by compiling trim_whitespace() and parse_mod() straight out of parse_config.c into a small harness with stubbed wlroots constants. Before the change code:64 and code:64+code:133 return 4294967295, after it they return ALT and ALT|LOGO. ALT, SUPER+SHIFT, NONE, SUPER+code:50, code:24 and bogus give the same result either way. I did not build the full compositor, and the file passes clang-format unchanged.

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