Minor fixes to socket support when used with LWIP

 - LWIP UDP sockets are always writable and they are not reported as
   such by select() (bug? featuyre?). TCP sockets are ok.
 - Always deliver MG_EV_POLL, whether we have other fd_events or not.
   Otherwise MG_EV_POLL would not be delivered at all to perma-writable
   UDP sockets (as they are on LWIP).
 - Ignore EAGAIN/EWOULDBLOCK errors while connecting, this happens
   during SSL handshake.

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