acl-plugin: add whitelisted ethertype mode (VPP-1163)

acl-plugin: add whitelisted ethertype mode (VPP-1163)

Currently, ACL plugin largely does not care about the
ethertypes other than 0x0800 (IPv4) and 0x86dd (IPv6),
the only exception being 0x0806 (ARP), which is
dealt with by the MACIP ACLs.

The other ethertypes in L2 mode are just let through.

This adds a new API message acl_interface_set_etype_whitelist,
which allows to flip the mode of a given interface
into "ethertype whitelist mode": the caller of this message
must supply the two lists (inbound and outbound) of the ethertypes
that are to be permitted, the rest of the ethertypes are
dropped.

The whitelisting for a given interface and direction takes
effect only when a policy ACL is also applied.

This operates on the same classifier node as the one used for
dispatching the policy ACL, thus, if one wishes for most of the
reasonable IPv4 deployments to continue to operate within
the whitelist mode, they must permit ARP ethertype (0x0806)

The empty list for a given direction resets the processing
to allow the unknown ethertypes. So, if one wants to just
permit the IPv4 and IPv6 and nothing else, one can add
their ethertypes to the whitelist.

Add the "show acl-plugin interface" corresponding outputs
about the whitelists, vat command, and unittests.

Change-Id: I4659978c801f36d554b6615e56e424b77876662c
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>