core: introduce unit_verify_contexts

Refuse WorkingDirectory=~ both in that and exec_invoke()
when dynamic user is used.

src/core/exec-invoke.c

@@ -3600,6 +3600,9 @@ static int acquire_home(const ExecContext *c, const char **home, char **ret_buf)
         if (!c->working_directory_home)
                 return 0;
 
+        if (c->dynamic_user)
+                return -EADDRNOTAVAIL;
+
         r = get_home_dir(ret_buf);
         if (r < 0)
                 return r;