Pure OCaml ARP table lookup - reads /proc/net/arp on Linux and arp -a on macOS/BSD

Add source stanzas to dune-project files

Fix monopam warnings about missing source stanzas and ca-certs parse error.

+1
+1
dune-project
··· 1 1 (lang dune 3.0) 2 2 (name arp) 3 + (source (uri https://tangled.org/gazagnaire.org/ocaml-arp)) 3 4 4 5 (generate_opam_files true) 5 6