Add SETCONTROL handling to config updater

This allows the config updater to filter SETCONTROL commands and set joy button defaults to the user's config, if a slot is available.

A few logic highlights:

• If keynum1 is unset AND keynum2 is set (happens sometimes), shift keynum2 down to keynum1.
• If a config'd button already exists, overwrite that pre-existing control -- same behavior as previously
• When forcing a default -- if THAT default button already exists, DON'T force the default

I'm 90% sure this code is okay. I'm tired to think of test cases right now -- will comment later.

This does generally work for me, but I didn't test with config'ing in-menu yet.