net/udp: Fix several issues using connected UDP sockets: sendto() should not...

net/udp:  Fix several issues using connected UDP sockets:  sendto() should not accept and address.  Normally send() should be used instead.  recvfrom() is no longer necessary.  recvfrom() and poll() should not reset to accept any addresses but should, instead, only accept inputs and events from the connected remore peer.  On of these issues was noted by 李桂丁 <liguiding@pinecone.net>.  Changes were made as summarized here: http://www.masterraghu.com/subjects/np/introduction/unix_network_programming_v1.3/ch08lev1sec11.html