Turns out --force is just in the way for most people, and at least for default
bindings it's easy to reset them.
Also, it makes :config-source fail when config.py contains keybindings.
Closes#3049
Before, we allowed :set to take multiple values, which often lead to confusing
error messages when a user forgot to quote the value.
Now, we instead have a dedicated :config-cycle command for that.
See #1840, #2794