vnet: create L2 feature arc infrastructure

vnet: create L2 feature arc infrastructure

This commit adds two new nodes in the L2
datapath in input and output direction respectively.

These nodes fork the traffic into three feature
arcs: ip4, ip6 and nonip, which later join
to continue the usual L2 processing.
The vnet_l2_feature_enable_disable() function
with the same signature as
vnet_feature_enable_disable() takes care of
enabling the L2 datapath feature bits as needed, when
the features are enabled/disabled.

Thus, L2 features may use the similar plumbing as
the L3 features enjoy.

Change-Id: I76877b3a92d794c492bff1622bb26acba05705b2
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>