netns-exec: add c program to execute commands in netns

c program allows executing commands in nb-bitcoind, nb-lnd, nb-liquidd
(the netns's needed for operator cli scripts).

pkgs/netns-exec/default.nix

@@ -0,0 +1,11 @@
+{ stdenv, pkgs }:
+
+stdenv.mkDerivation {
+  name = "netns-exec";
+  buildInputs = [ pkgs.libcap ];
+  src = ./src;
+  installPhase = ''
+    mkdir -p $out
+    cp main $out/netns-exec
+  '';
+}