nixos/filesystems: make supportedFilesystems an attrset

this lets us *dis*able filesystem explicitly, as is required by e.g. the
zfs-less installer images. currently that specifically is only easily
possible by adding an overlay that stubs out `zfs`, with the obvious
side-effect of also removing tooling that could run without the kernel
module loaded.

nixos/modules/tasks/filesystems.nix

@@ -246,10 +246,23 @@ in
     };
 
     boot.supportedFilesystems = mkOption {
-      default = [ ];
-      example = [ "btrfs" ];
-      type = types.listOf types.str;
-      description = lib.mdDoc "Names of supported filesystem types.";
+      default = { };
+      example = lib.literalExpression ''
+        {
+          btrfs = true;
+          zfs = lib.mkForce false;
+        }
+      '';
+      type = types.coercedTo
+        (types.listOf types.str)
+        (enabled: lib.listToAttrs (map (fs: lib.nameValuePair fs true) enabled))
+        (types.attrsOf types.bool);
+      description = lib.mdDoc ''
+        Names of supported filesystem types, or an attribute set of file system types
+        and their state. The set form may be used together with `lib.mkForce` to
+        explicitly disable support for specific filesystems, e.g. to disable ZFS
+        with an unsupported kernel.
+      '';
     };
 
     boot.specialFileSystems = mkOption {