nfs-ganesha: add tools output for mount.9P helper

The nfs-ganesha mount.9P helper is a standalone shell script that makes
some aspects of mounting a 9P filesystem easier, e.g. instead of:

mount -t 9p 192.168.10.2 /mnt -oaname=/home

it allows the familiar syntax:

mount -t 9P 192.168.10.2:/home /mnt

As the mount helper is independent of the server, it's added as
separate output.

pkgs/servers/nfs-ganesha/default.nix

···
 stdenv.mkDerivation rec {
   pname = "nfs-ganesha";
   version = "5.7";
+  outputs = [ "out" "tools" ];
 
   src = fetchFromGitHub {
     owner = "nfs-ganesha";
···
     nfs-utils
   ];
 
+  postPatch = ''
+    substituteInPlace src/tools/mount.9P --replace "/bin/mount" "/usr/bin/env mount"
+  '';
+
   postFixup = ''
     patchelf --add-rpath $out/lib $out/bin/ganesha.nfsd
   '';
 
+  postInstall = ''
+    install -Dm755 $src/src/tools/mount.9P $tools/bin/mount.9P
+  '';
+
   meta = with lib; {
     description = "NFS server that runs in user space";
     homepage = "https://github.com/nfs-ganesha/nfs-ganesha/wiki";
···
     platforms = platforms.linux;
     license = licenses.lgpl3Plus;
     mainProgram = "ganesha.nfsd";
+    outputsToInstall = [ "out" "tools" ];
   };
 }