[franck-messaoudi]

This adds an element group, elements/lisp, that implements the IPv4 parts of
LISP (RFC 9300 data plane,
RFC 9301 control plane), plus a
Follow Me Cloud extension that moves virtual machines between data centers
and updates the LISP mappings so their traffic follows them.

LISP splits an IP address into an endpoint identifier (EID), which stays the
same when a host moves, and a routing locator (RLOC), the address of the site
router. Tunnel routers (xTRs) encapsulate traffic between sites and learn
EID-to-RLOC mappings from a Map-Server / Map-Resolver.

The group is off by default (--enable-lisp, or --enable-all-elements) and
touches nothing outside elements/lisp/, include/clicknet/lisp.h,
conf/lisp/, test/lisp/, configure.in/configure, .gitignore and
AUTHORS.

Elements

Role	Elements
Data plane	LISPEncap, LISPDecap
Classification	LISPClassifierxTR, LISPClassifierMS, LISPClassifierFMCC
ITR	LISPMapCache, LISPMapRequester
ETR	LISPEtrDatabase, LISPMapReplier, LISPMapRegistrar
Map-Server / Map-Resolver	LISPMappingDB, LISPMapServer, LISPMapResolver
Follow Me Cloud	FMCCDatabase, FMCCManager, FMCCMigrator, LISPMigrationUpdater, LISPVMCounter

Wire formats are in include/clicknet/lisp.h (plain C, byte-exact, network
byte order).

What is covered

• Data plane: LISP/UDP/IP encapsulation with nonce, locator-status-bits,
  instance ID, map-versions, and a flow-hash UDP source port.
• Control plane: Map-Request (plain or in an Encapsulated Control Message),
  Map-Reply, Negative Map-Reply, Map-Register and Map-Notify with
  HMAC-SHA-256-128 authentication, Solicit-Map-Request, per-site
  authorization on the Map-Server, registration expiry.
• Map-cache with longest-prefix match and locator sets chosen by priority,
  then weight per flow.
• Follow Me Cloud: a Map-Migrate message, carried as an RFC 8113 shared
  extension message (type 15, provisional sub-type), optionally
  HMAC-authenticated.

Not supported yet: IPv6 EIDs and RLOCs, PITR/PETR, RLOC-probing, LISP-SEC,
and the Linux kernel module (the elements are tested at userlevel only).

Testing

./configure --disable-linuxmodule --enable-lisp
make -j"$(nproc)"
make check            # includes the 11 tests in test/lisp/

• make check: 183 successes, 0 failures (gcc 12.3, Ubuntu 22.04).
• Builds without warnings in the LISP files with gcc 12.3 and with clang 14
  (--enable-all-elements -std=gnu++11, as in CI), single-threaded and with
  --enable-user-multithread (4-thread stress test).
• AddressSanitizer clean on the test configurations.
• Every message the elements send decodes in Wireshark with no malformed
  field (test/lisp/LISPWire-01.clicktest, run when tshark is installed).
• Each commit builds on its own.
• GitHub CI: all 6 jobs (gcc and clang; vanilla, DPDK and netmap) pass on
  this branch together with the CI fix in GitHub CI: Use a Python version available on current runners #512 (the current workflow fails
  at "Set up Python" on every branch, including master, because Python 3.7
  is no longer available on the runners):
  https://github.com/franck-messaoudi/click/actions/runs/39845537158

conf/lisp/xtr1_xtr2_fmcc.click runs two xTRs, a Map-Server/Map-Resolver and
the FMCC controller in one process; elements/lisp/LISP_GUIDE.md has a quick
start for it.

Notes for review

• Commits follow the build order: wire formats, shared code, data plane, ITR,
  ETR, Map-Server/Map-Resolver, Follow Me Cloud, tests, documentation,
  configure, AUTHORS.
• configure was updated with only the hunks that the new
  ELEMENTS_ARG_ENABLE(lisp, ...) line generates, so the diff stays at 13
  lines; regenerating it with your usual Autoconf gives the same option.
• Every element has an =c/=s/=d/=h documentation comment; the
  internal API also has Doxygen comments (doxygen elements/lisp/Doxyfile).
• The Map-Migrate sub-type (0xFC0) is a placeholder until a value is
  registered with IANA.