httpev: add http_bind function

setup http_server with http_config binded to systemd socket (by default)
or to provided addr (host:port)

httpev.ml

@@ -1119,6 +1119,21 @@ let setup_lwt config answer =
 let server_lwt config answer =
   Lwt_main.run @@ setup_lwt config answer
 
+let http_bind addr http_config =
+  match addr, Systemd.Daemon.listen_fds_lwt () with
+  | None, [] ->
+    log#error "bind not provided and no systemd socket available";
+    exit 1
+  | None, fd :: fds ->
+    log#info "starting httpev in systemd mode";
+    if fds <> [] then
+      log#warn "more than one fd is provided by systemd, only the first one is used and the other ones are ignored";
+    setup_fd_lwt fd http_config, "systemd socket"
+  | Some bind, _ ->
+    let connection = Nix.sockaddr_of_string bind in
+    let signature = sprintf "listen %s" (Nix.show_addr connection) in
+    setup_lwt { http_config with connection }, signature
+
 module Answer = struct
 
 let return ?(status=`Ok) ?(extra=[]) ~typ data : [> `Body of reply_status reply' ] Lwt.t =