TE: MPLS: if_mpe(4): refactoring

Now, if_mpe(4) is capable to tx MPLS label tagged PDU by
proxyfied interface.

OSI-L2/VPN capability is still untested...

Found and terminated some bugs due to OSI-L2/VPN,
during testing. I've removed mpls_link_handoff()
and replaced by

    switch (ifp->if_type) {
    case IFT_ETHER:
    case IFT_VETHER:
    case IFT_FDDI:
        if ((m = mpls_shim_pop(m)) == NULL)
            break;

            m_adj(m, MPLS_CWLEN);
                            /* FALLTHROUGH */
    case IFT_MPLS:
        (*ifp->if_input)(ifp, m);
        break;
    default:
        break;
    }

but testing still continues, please stand by.
Further, I'm starting over to rewrite some parts
of README and coninue describing FTN.