evdev: Extend EVIOCGRAB ioctl scope to cover kbd interface

of various keyboard drivers.
EVIOCGRAB ioctl execution on /dev/input/event# device node gains
exclusive access to this device to caller. It is used mostly for
development purposes and remote control software. See e.g.
https://reviews.freebsd.org/D30020 which is the reason of creation
of this change.
Keyboard grabbing is disabled in KDB and during panics.

MFC with: 4a0db5e2920c
Tested by: corvink
Differential revision: https://reviews.freebsd.org/D30542