nixos/systemd: Allow creation of unit directories

This patch allows creation of files like
/etc/systemd/system/user-.slice.d/limits.conf with

    systemd.units."user-.slice.d/limits.conf" = {
      text = ''
        [Slice]
        CPUAccounting=yes
        CPUQuota=50%
      '';
    };

which previously threw an error

Also renames the systemd-unit-path test to sytsemd-misc, and extends it to
test that `systemd.units` can handle directories. In this case we make
sure that resource limits specified in user slices apply.

nixos/lib/systemd-lib.nix

@@ -23,8 +23,9 @@ in rec {
           inherit (unit) text;
         }
         ''
-          mkdir -p $out
-          echo -n "$text" > $out/${shellEscape name}
+          name=${shellEscape name}
+          mkdir -p "$out/$(dirname "$name")"
+          echo -n "$text" > "$out/$name"
         ''
     else
       pkgs.runCommand "unit-${mkPathSafeName name}-disabled"
@@ -32,8 +33,9 @@ in rec {
           allowSubstitutes = false;
         }
         ''
-          mkdir -p $out
-          ln -s /dev/null $out/${shellEscape name}
+          name=${shellEscape name}
+          mkdir -p "$out/$(dirname "$name")"
+          ln -s /dev/null "$out/$name"
         '';
 
   boolValues = [true false "yes" "no"];