METEORSSIGNAL ioctl. Applications use this ioctl with the value
METEOR_SIG_MODE_MASK (0xFFFF0000, -65536) to reset signal delivery,
but revision 1.126 caused the driver to return EINVAL in this case.
Interestingly, the same METEORSSIGNAL ioctl in the meteor driver uses
0 to reset signal delivery.
This commit allows METEOR_SIG_MODE_MASK as a synonym for 0 in the
bktr driver, and restructures the code a bit so that it is otherwise
identical between the bktr and meteor drivers.