Introduce a Linux FHS run mode

The idea is that distro native packages can use this runmode without
having to patch the code base. This is still not complete since there's
no automatic detection of when to use this.

Author: ekohl

lib/puppet/util/run_mode.rb

@@ -103,6 +103,53 @@ def vendor_module_dir
       end
     end
 
+    class LinuxFHSRunMode < RunMode
+      def conf_dir
+        # TODO: ENV['XDG_CONFIG_HOME']
+        which_dir("/etc/puppet", "~/.config/puppet")
+      end
+
+      def code_dir
+        File.join(conf_dir, 'code')
+      end
+
+      def var_dir
+        # TODO: ENV['XDG_DATA_HOME']
+        which_dir("/var/lib/puppet", "~/.local/share/puppet")
+      end
+
+      def public_dir
+        # TODO: ENV['XDG_CACHE_HOME']
+        which_dir("/var/cache/puppet/public", "~/.cache/puppet/public")
+      end
+
+      def run_dir
+        # TODO ENV['XDG_RUNTIME_DIR']
+        which_dir("/run/puppet", "/run/user/#{Etc.getpwuid.uid}")
+      end
+
+      def log_dir
+        # TODO ENV['XDG_STATE_HOME']
+        which_dir("/var/log/puppet", "~/.local/state/puppet/logs")
+      end
+
+      def pkg_config_path
+        # automatically picked up
+      end
+
+      def gem_cmd
+        '/usr/bin/gem'
+      end
+
+      def common_module_dir
+        '/usr/share/puppet/modules'
+      end
+
+      def vendor_module_dir
+        '/usr/share/puppet/vendor_modules'
+      end
+    end
+
     class WindowsRunMode < RunMode
       def conf_dir
         which_dir(File.join(windows_common_base("puppet/etc")), "~/.puppetlabs/etc/puppet")