if_tap: Some small tweaks and cleanups

if_tap: Some small tweaks and cleanups

* Consistently use the TAP define instead of "tap", also remove the
  CDEV_NAME define.

* Consistently use "tap_if" instead of "arpcom.ac_if".

* Better name the kqueue functions, and move the structs to the beginning.

* Reorder to better group related functions.

* Explicitly include the required <sys/queue.h> header.

* Use variable "sc" instead of "tp" to refer to "struct tap_softc".

* Staticize the bitmap variable.

* Use the variable unit = minor(dev) more.

* Improve the debug messages.

* Various minor style tweaks and cleanups.