Improve logrotate configuration file

- Avoid changing file permissions
- Make sure all log files are reopened
- Remove non-sense prerotate script
- Send SIGUSR1 to all nghttpx processes

contrib/nghttpx-logrotate

 /var/log/nghttpx/*.log {
   weekly
-        missingok
   rotate 52
+  missingok
   compress
   delaycompress
   notifempty
-        create 0640 www-data adm
-        sharedscripts
-        prerotate
-                if [ -d /etc/logrotate.d/httpd-prerotate ]; then \
-                        run-parts /etc/logrotate.d/httpd-prerotate; \
-                fi \
-        endscript
   postrotate
-                [ -s /run/nghttpx.pid ] && kill -USR1 `cat /run/nghttpx.pid`
+    killall -USR1 nghttpx 2> /dev/null || true
   endscript
 }