nixos/fileSystems: add enable option

Add the ability to disable filesystems set in a nixos config (enabled by
default). This makes the submodules under `fileSystems` behave more like
submodules in other nixos modules.

nixos/modules/tasks/filesystems.nix

@@ -30,6 +30,10 @@ let
   coreFileSystemOpts = { name, config, ... }: {
 
     options = {
+      enable = mkEnableOption "the filesystem mount" // {
+        default = true;
+      };
+
       mountPoint = mkOption {
         example = "/mnt/usb";
         type = nonEmptyWithoutTrailingSlash;
@@ -222,6 +226,7 @@ in
         }
       '';
       type = types.attrsOf (types.submodule [coreFileSystemOpts fileSystemOpts]);
+      apply = lib.filterAttrs (_: fs: fs.enable);
       description = ''
         The file systems to be mounted.  It must include an entry for
         the root directory (`mountPoint = "/"`).  Each
@@ -268,6 +273,7 @@ in
     boot.specialFileSystems = mkOption {
       default = {};
       type = types.attrsOf (types.submodule coreFileSystemOpts);
+      apply = lib.filterAttrs (_: fs: fs.enable);
       internal = true;
       description = ''
         Special filesystems that are mounted very early during boot.