Add evdev support to ukbd driver

event generation is disabled by default in favour of kbdmux. This
behavoiur is controlled by kern.evdev.rcpt_mask sysctl, bit 3 should
be set to give priority to hw over mux

Submitted by: Vladimir Kondratiev <wulf@cicgroup.ru>
Reviewed by: hans
Differential Revision: https://reviews.freebsd.org/D7957