[server] fix FD_READ signaling for datagram sockets

[Damjan Jovanovic]

Some (IMO poorly written) apps expect that when they receive a FD_READ
message, a subsequent attempt to read data from that datagram socket
will never fail, not even with an innocent EWOULDBLOCK.

Changelog:
* When polling a datagram socket says there is data to read, verify
that before delivering the FD_READ message to the application
* Closes #5774 (udp traffic stops after a while with emule, uTorrent)

Damjan Jovanovic
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