acpi_ibm(4): new radio kill switch (readonly) sysctl

[Rong-en Fan]

As pointed out by Henrik Brix Andersen, I adds a sysctl entry
that shows the status of radio kill switch found on some ThinkPad:

http://people.freebsd.org/~rafan/acpi_ibm_killswitch.diff

dev.acpi_ibm.0.killswitch = 0 means the switch is off. It seems that
no acpi event will be generated when the value changes (actually,
my x60 does not generate any events when I presses FN+something).
Otherwise, we can hook it in devd.conf and remove wireless driver when
kill switch is on...

Any comments?

Regards,
Rong-En Fan