tree-wide: print warning in a failure case of make_null_stdio() (#4320)

The make_null_stdio() may fail. Let's check its result and print
warning message instead of keeping silence.

src/core/main.c

@@ -1532,7 +1532,8 @@ int main(int argc, char *argv[]) {
                  * need to do that for user instances since they never log
                  * into the console. */
                 log_show_color(colors_enabled());
-                make_null_stdio();
+                if (make_null_stdio() < 0)
+                        log_warning_errno(errno, "Failed to redirect standard streams to /dev/null: %m");
         }
 
         /* Initialize default unit */

src/udev/udevd.c

@@ -1739,7 +1739,9 @@ int main(int argc, char *argv[]) {
 
                 /* connect /dev/null to stdin, stdout, stderr */
                 if (log_get_max_level() < LOG_DEBUG)
-                        (void) make_null_stdio();
+                        if (make_null_stdio() < 0)
+                                log_warning_errno(errno, "Failed to redirect standard streams to /dev/null: %m");
+
 
                 pid = fork();
                 switch (pid) {