UPDATE: 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. Due to RTF_POP operation, if
not Bos, some minor changes in mpls_forward:

    if (hasbos == 0) {
        m = mpls_shim_pop(m);
        if ((ifp->if_type == IFT_MPLS)
            || (ifp->if_bridge != NULL)) {
            shim = mtod(m, struct shim_hdr *);
            mpls_rtfree(ftn);
            continue;
        }
        shim = mtod(m, struct shim_hdr *);
        satosmpls(ftn->ftn_gw)->smpls_label =
            shim->shim_label & MPLS_LABEL_MASK;
            break;
        }
    }

further, 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:
        goto out;
    }

but testing still continues, please stand by.

Further, I'm starting over to rewrite some parts
of README and coninue describing FTN.