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

Fixes #10270.

nixos/modules/config/system-path.nix

···
       pkgs.xz
       pkgs.less
       pkgs.libcap
-      pkgs.man
       pkgs.nano
       pkgs.ncurses
       pkgs.netcat
···
         "/info"
         "/lib" # FIXME: remove
         #"/lib/debug/.build-id" # enables GDB to find separated debug info
-        "/man"
         "/sbin"
         "/share/applications"
         "/share/desktop-directories"
···
         "/share/emacs"
         "/share/icons"
         "/share/info"
-        "/share/man"
         "/share/menus"
         "/share/mime"
         "/share/nano"

nixos/modules/module-list.nix

···
   ./programs/command-not-found/command-not-found.nix
   ./programs/dconf.nix
   ./programs/environment.nix
+  ./programs/freetds.nix
   ./programs/ibus.nix
   ./programs/kbdlight.nix
   ./programs/light.nix
+  ./programs/man.nix
   ./programs/nano.nix
   ./programs/screen.nix
   ./programs/shadow.nix
···
   ./programs/uim.nix
   ./programs/venus.nix
   ./programs/wvdial.nix
-  ./programs/freetds.nix
   ./programs/xfs_quota.nix
   ./programs/zsh/zsh.nix
   ./rename.nix

nixos/modules/programs/man.nix

···
+{ 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" ];
+
+  };
+
+}

nixos/modules/services/misc/nixos-manual.nix

···
 
     system.build.manual = manual;
 
-    environment.systemPackages = [ manual.manpages manual.manual help ];
+    environment.systemPackages =
+      [ manual.manual help ]
+      ++ optional config.programs.man.enable manual.manpages;
 
     boot.extraTTYs = mkIf cfg.showManual ["tty${cfg.ttyNumber}"];