various: add %m in messages

Sometimes we want to suppress strerror() message because the are providing
something better. But in those cases, it seems it was just forgotten.

src/userdb/userwork.c

@@ -556,7 +556,7 @@ static int run(int argc, char *argv[]) {
                                         return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Parent already died?");
 
                                 if (kill(parent, SIGUSR2) < 0)
-                                        return log_error_errno(errno, "Failed to kill our own parent.");
+                                        return log_error_errno(errno, "Failed to kill our own parent: %m");
                         }
                 }