keybind: refactor update_keycodes_iter() to reduce nesting

keybind: refactor update_keycodes_iter() to reduce nesting

update_keycodes_iter() currently has 4(!) levels of nested loops, which
makes the logic (especially the break/continue statements) difficult to
understand. The logic also appears to continue looping uselessly after
a given keycode has already been added to a keybind.

Refactor by adding some small utility functions:

- keybind_contains_keycode()
- keybind_contains_keysym()
- keybind_contains_any_keysym()

No functional change intended.