* Add SIOCGI2C driver ioctl used to retrieve i2c info.
* Convert ixgbe to use this ioctl
* Convert ifconfig to use generic i2c handler for  "ix" interfaces.

Approved by:	Eric Joyner (ixgbe part)
MFC after:	2 weeks
Sponsored by:	Yandex LLC
Use a unit number allocator to provide suitable st_dev and st_ino values
for POSIX shared memory descriptors.  The implementation is similar to
that used for pipes.

MFC after:	1 week