dev: Rename EtherTap to be EtherTapStub.

dev: Rename EtherTap to be EtherTapStub.

The EtherTap object is going to be reworked so that it connects to a tap
device directly, but it's worthwhile to still be able to use the m5tap
utility (util/tap) to send/receive packets on systems which don't support
tap but do support the pcap API. It can also be used to replay ethernet
frames, to capture the ethernet frames coming from gem5 for analysis, to
programmatically consume and/or generate the frames, or even to forward
them to/from a remote system.

Change-Id: Ic7bd763d86cd913ac373dd10a8d6d1fc6b35f95a
Reviewed-on: https://gem5-review.googlesource.com/3644
Reviewed-by: Nathan Binkert <nate@binkert.org>
Maintainer: Nathan Binkert <nate@binkert.org>