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/bitcoin-cash-node.nix

@@ -0,0 +1,48 @@
+{ pkgs }:
+
+pkgs.stdenv.mkDerivation rec {
+    pname = "bitcoin-cash-node";
+    version = "v28.0.0";
+    src = fetchGit {
+        url = "https://gitlab.com/bitcoin-cash-node/bitcoin-cash-node";
+        ref = "refs/tags/${version}";
+    };
+    nativeBuildInputs = [
+        pkgs.cmake
+        pkgs.ninja
+        pkgs.help2man
+        pkgs.python3
+    ];
+    buildInputs = [
+        pkgs.boost
+        pkgs.libevent
+        pkgs.db
+        pkgs.gmp
+        pkgs.openssl
+        pkgs.miniupnpc
+        pkgs.libnatpmp
+        pkgs.zeromq
+        pkgs.zlib
+    ];
+   cmakeFlags = [
+      "-GNinja"
+      "-DBUILD_BITCOIN_QT=OFF"
+      "-DVERBOSE_CONFIGURE=ON"
+   ];
+    doCheck = false;
+    postConfigure = ''
+        chmod ug+x config/run_native_cmake.sh
+        chmod ug+x src/secp256k1/build_native_gen_context.sh
+        sed -e 's@/usr/bin/env python3@${pkgs.python3}/bin/python3@' -i ../cmake/utils/gen-ninja-deps.py
+        sed -e 's@/usr/bin/env bash@${pkgs.bash}/bin/bash@' -i doc/man/gen-doc-man-footer.sh
+        sed -e 's@/usr/bin/env bash@${pkgs.bash}/bin/bash@' -i doc/man/gen-doc-man.sh
+    '';
+    postInstall= ''
+        rm -f $out/bin/*
+        install -v --mode=755 src/bitcoind $out/bin/bitcoind-bchn
+        install -v --mode=755 src/bitcoin-cli $out/bin/bitcoin-cli-bchn
+        rm -f $out/share/man/man1/bitcoin-{tx,seeder}*
+        mv $out/share/man/man1/bitcoind.1 $out/share/man/man1/bitcoind-bchn.1
+        mv $out/share/man/man1/bitcoin-cli.1 $out/share/man/man1/bitcoin-cli-bchn.1
+    '';
+}