Release tcpip 4.1.0

[hannesm]

[new release] tcpip (4.1.0)

CHANGES:

• Revert "Ipv4.Fragments use a Lru.M.t instead of Lru.F.t" (Revert "Ipv4.Fragments: use a mutable LRU cache (Lru.M.t instead of mutable cache : Lru.F.t):" mirage/mirage-tcpip#423 by @hannesm)
  A Lru.M.t allocates a Hashtbl.t of size = capacity (= 256 * 1024 in our case),
  this leads to excessive ~2MB memory consumption for each Fragment cache,
  reported by @xaki23 in serious memory issues since mirage-3.7 update mirage/qubes-mirage-firewall#93
• use SOCK_RAW for an ICMP socket in the unix sockets API (previously used
  SOCK_DGRAM which did not work)
  reported by @justinc1 in Unix.EACCES error when running ping example mirage/mirage-tcpip#358, fixed in stack-unix: use a RAW socket for icmp (fixes #358) mirage/mirage-tcpip#424 by @hannesm
• tcp is now compatible with lwt >= 5.0.0 (where Lwt.async requires a function
  of (unit -> unit Lwt.t) (Start preparation for Lwt.async demanding unit Lwt.t mirage/mirage-tcpip#370 tcp: ignore result of xmit, continues #310, see #392 mirage/mirage-tcpip#425 @cfcs @hannesm, issue Should q.xmit return value be ignored? mirage/mirage-tcpip#392 @emillon)
• Add a dependency on dune-configurator to support dune 2.0.0 (travis: no need for extra remote anymore, cohttp was released mirage/mirage-tcpip#421 @avsm)

[hannesm]

the test failure on macOS is expected (been there since several releases, still unclear why it fails -- someone with a mac and interest in the tcpip package looking into that would be appreciated)

[kit-ty-kate]

Thanks!