logind: port over to use scopes+slices for all cgroup stuff

In order to prepare things for the single-writer cgroup scheme, let's
make logind use systemd's own primitives for cgroup management.

Every login user now gets his own private slice unit, in which his sessions
live in a scope unit each. Also, add user@$UID.service to the same
slice, and implicitly start it on first login.

src/shared/unit-name.c

@@ -565,3 +565,30 @@ UnitType unit_name_to_type(const char *n) {
 
         return unit_type_from_string(e + 1);
 }
+
+int build_subslice(const char *slice, const char*name, char **subslice) {
+        char *ret;
+
+        assert(slice);
+        assert(name);
+        assert(subslice);
+
+        if (streq(slice, "-.slice"))
+                ret = strappend(name, ".slice");
+        else {
+                char *e;
+
+                e = endswith(slice, ".slice");
+                if (!e)
+                        return -EINVAL;
+
+                ret = new(char, (e - slice) + 1 + strlen(name) + 6 + 1);
+                if (!ret)
+                        return -ENOMEM;
+
+                stpcpy(stpcpy(stpcpy(mempcpy(ret, slice, e - slice), "-"), name), ".slice");
+        }
+
+        *subslice = ret;
+        return 0;
+}

src/shared/unit-name.h

@@ -99,3 +99,5 @@ int unit_name_from_dbus_path(const char *path, char **name);
 
 char *unit_name_mangle(const char *name);
 char *unit_name_mangle_with_suffix(const char *name, const char *suffix);
+
+int build_subslice(const char *slice, const char*name, char **subslice);