Nix support improvements

Quick Start for BTC:

    $ git clone https://github.com/mmgen/mmgen-wallet
    $ cd mmgen-wallet
    $ nix-shell --pure nix

Enable altcoins and additional packages:

    $ mkdir -p ~/.mmgen
    $ cp nix/user-packages.nix ~/.mmgen
    # ... edit ~/.mmgen/user-packages.nix as required ...
    $ nix-shell --pure nix

For NixOS installation and other information, see:

    nix/README

nix/go-ethereum.nix

@@ -0,0 +1,51 @@
+## adapted from go-ethereum.nix in nixpkgs repository
+
+{
+    pkgs,
+    lib,
+    stdenv,
+    buildGoModule,
+    # nixosTests,
+    tag_version,
+    vendor_hash,
+}:
+
+buildGoModule {
+    pname = "go-ethereum";
+    version = tag_version;
+
+    src = fetchGit {
+        url = "https://github.com/ethereum/go-ethereum.git";
+        ref = "refs/tags/${tag_version}";
+    };
+
+    proxyVendor = false;
+
+    vendorHash = vendor_hash;
+
+    doCheck = false;
+
+    subPackages = [ "cmd/geth" ];
+
+    ## Following upstream: https://github.com/ethereum/go-ethereum/blob/v1.11.6/build/ci.go#L218
+    tags = [ "urfave_cli_no_docs" ];
+
+    ## Fix for usb-related segmentation faults on darwin
+    propagatedBuildInputs = lib.optionals stdenv.hostPlatform.isDarwin [
+       pkgs.libobjc
+       pkgs.IOKit
+    ];
+
+    # passthru.tests = { inherit (nixosTests) geth; };
+
+    meta = with lib; {
+        homepage = "https://geth.ethereum.org/";
+        description = "Official golang implementation of the Ethereum protocol";
+        license = with licenses; [
+            lgpl3Plus
+            gpl3Plus
+        ];
+        maintainers = with maintainers; [ RaghavSood ];
+        mainProgram = "geth";
+    };
+}