cgroup: change cg_unified() to possibly return errors again

We use our cgroup APIs in various contexts, including from our libraries
sd-login, sd-bus. As we don#t control those environments we can't rely
that the unified cgroup setup logic succeeds, and hence really shouldn't
assert on it.

This more or less reverts 415fc41cea.

src/basic/cgroup-util.h

@@ -240,9 +240,9 @@ int cg_kernel_controllers(Set *controllers);
 
 bool cg_ns_supported(void);
 
-bool cg_all_unified(void);
-bool cg_hybrid_unified(void);
-bool cg_unified(const char *controller);
+int cg_all_unified(void);
+int cg_hybrid_unified(void);
+int cg_unified(const char *controller);
 int cg_unified_flush(void);
 
 bool cg_is_unified_wanted(void);