core: simplify cg_[all_]unified()

cg_[all_]unified() test whether a specific controller or all controllers are on
the unified hierarchy.  While what's being asked is a simple binary question,
the callers must assume that the functions may fail any time, which
unnecessarily complicates their usages.  This complication is unnecessary.
Internally, the test result is cached anyway and there are only a few places
where the test actually needs to be performed.

This patch simplifies cg_[all_]unified().

* cg_[all_]unified() are updated to return bool.  If the result can't be
  decided, assertion failure is triggered.  Error handlings from their callers
  are dropped.

* cg_unified_flush() is updated to calculate the new result synchrnously and
  return whether it succeeded or not.  Places which need to flush the test
  result are updated to test for failure.  This ensures that all the following
  cg_[all_]unified() tests succeed.

* Places which expected possible cg_[all_]unified() failures are updated to
  call and test cg_unified_flush() before calling cg_[all_]unified().  This
  includes functions used while setting up mounts during boot and
  manager_setup_cgroup().

src/basic/cgroup-util.h

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