Factor out "man" into a separate module and add "man" outputs to system.path

Fixes #10270.

nixos/modules/programs/man.nix

@@ -0,0 +1,30 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+
+  options = {
+
+    programs.man.enable = mkOption {
+      type = types.bool;
+      default = true;
+      description = ''
+        Whether to enable manual pages and the <command>man</command> command.
+      '';
+    };
+
+  };
+
+
+  config = mkIf config.programs.man.enable {
+
+    environment.systemPackages = [ pkgs.man ];
+
+    environment.pathsToLink = [ "/share/man" ];
+
+    environment.outputsToLink = [ "man" ];
+
+  };
+
+}