keymap: More robust state machine

keymap: More robust state machine

Some drivers, like thinkpad_acpi, do not send a scan code at all (for known
keys), and some send the key code first, then the scan code. Implement a better
state machine which acceps them in any order and wait until a SYN event. If the
driver does not send SYN events, keymap will also handle this and print out
that fact.

Thanks to Seth Forshee for pointing out how this really works!

https://launchpad.net/bugs/702407