lxd/ip/neigh: Rework Show to return a useful struct rather than just a raw string
lxd/ip/neigh: Rework Show to return a useful struct rather than just a raw string

In doing so, move some existing state constants into IP package.

Add support for optional filtering by MAC.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

stgraber/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd