Improves the handling of port 0
Improves the handling of port 0

Usually port 0 means that the OS should pick any free port. After
listen() has been called the picked port can be determined and used
from then on.

silvansky/GCDWebServer,edmstudio/GCDWebServer,fluffyemily/GCDWebServer,zh-XN/GCDWebServer,TimOliver/GCDWebServer,liuchungui/GCDWebServer,hushifei/GCDWebServer,aoman89757/GCDWebServer,donpark/GCDWebServer,fluffyemily/GCDWebServer,Coppertino/GCDWebServer,MinorUncle/GCDWebServer,CrossWaterBridge/GCDWebServer,boherna/GCDWebServer,ddnbgroup/GCDWebServer,hushifei/GCDWebServer,1yvT0s/GCDWebServer,sagmor/GCDWebServer,CrossWaterBridge/GCDWebServer,8fit/GCDWebServer,cnbin/GCDWebServer,fluffyemily/GCDWebServer,zh-XN/GCDWebServer,ddnbgroup/GCDWebServer,liuchungui/GCDWebServer,Coppertino/GCDWebServer,zh-XN/GCDWebServer,pollarm/GCDWebServer,liuchungui/GCDWebServer,swisspol/SANDBOX,silvansky/GCDWebServer,sagmor/GCDWebServer,1yvT0s/GCDWebServer,edmstudio/GCDWebServer,swisspol/SANDBOX,CrossWaterBridge/GCDWebServer,1yvT0s/GCDWebServer,MinorUncle/GCDWebServer,MinorUncle/GCDWebServer,pollarm/GCDWebServer,hushifei/GCDWebServer,TimOliver/GCDWebServer,TimOliver/GCDWebServer,palmin/GCDWebServer,CrossWaterBridge/GCDWebServer,tipbit/GCDWebServer,palmin/GCDWebServer,cnbin/GCDWebServer,aoman89757/GCDWebServer,donpark/GCDWebServer,swisspol/SANDBOX,tipbit/GCDWebServer,aoman89757/GCDWebServer,ddnbgroup/GCDWebServer,aoman89757/GCDWebServer,zh-XN/GCDWebServer,donpark/GCDWebServer,ddnbgroup/GCDWebServer,hushifei/GCDWebServer,boherna/GCDWebServer,sagmor/GCDWebServer,8fit/GCDWebServer,tipbit/GCDWebServer,pollarm/GCDWebServer,cnbin/GCDWebServer,swisspol/SANDBOX,boherna/GCDWebServer,palmin/GCDWebServer,silvansky/GCDWebServer,8fit/GCDWebServer,cnbin/GCDWebServer,palmin/GCDWebServer,8fit/GCDWebServer,fluffyemily/GCDWebServer,TimOliver/GCDWebServer,silvansky/GCDWebServer,Coppertino/GCDWebServer,MinorUncle/GCDWebServer,Coppertino/GCDWebServer,liuchungui/GCDWebServer,donpark/GCDWebServer,boherna/GCDWebServer,1yvT0s/GCDWebServer,edmstudio/GCDWebServer,pollarm/GCDWebServer,tipbit/GCDWebServer,edmstudio/GCDWebServer,sagmor/GCDWebServer