Drop KNI library

[AltraMayor]

KNI is no longer part of DPDK. We need to find a replacement. Possible candidates suggested in the commit that removed KNI are virtio_user, tap, and XDP drivers.

The positive byproduct of no longer relying on KNI is to address issues #570 and #585.

[AltraMayor]

DPDK's developers implicitly settled with virtio_user. The minutes of the technical board meeting on January 13th, 2021, describe virtio_user as "the most promising interface". Stephen Hemminger reported on virtio_user's superior performance on DPDK's mailing list on January 13th, 2021. The limitations of virtio_user, namely, lack of support for the kernel to change MAC, MTU, RX mode, and admin status are not features needed in Gatekeeper; in fact, Gatekeeper favors turning off these features since the parameters of the virtual NIC must be in line with the underlying physical interfaces in Gatekeeper. Finally, DPDK's manual has a how-to for integrating virtio_user.

The paper VIRTIO-USER: A New Versatile Channel for Kernel-Bypass Networks describes the design and evaluation of virtio_user. Virtio is a standardized interface originally designed to improve access to virtual devices by virtual machines. virtio_user leverages this standard to reach higher performance and reuse the kernel modules that implement the standard.

[AltraMayor]

Pull request #678 replaced DPDK's KNI library with virtio-user. This transition brings many benefits, such as

1. closing issues closes Avoid duplicate #define of KNI_FIFO_COUNT_MAX #481, Running tcpdump on kni interfaces brings BGP sessions down #570, and PANIC in rd_fill_getroute_reply #585;
2. simplifying the code of the CPS block. Pull request cps: replace DPDK's KNI library with virtio-user #678 dropped 1,422 lines while adding back only 656 lines;
3. eliminating the need to call fork() to set the parameters of the KNI interfaces which eliminates the need for the CPS parameter num_attempts_kni_link_set; and
4. dropping CPS' ELF library since the kernel automatically loads and unloads the kernel modules associated with Virtio.