Detect client EOF properly. Handle EWOULDBLOCK.

Client disconnection appears as a read of 0 bytes.
Without a test for this, dump1090 continues to poll that client forever.

Also, read() may return EWOULDBLOCK as well as EAGAIN
for "no data right now", so handle that.

I don't know if there is an equivalent Win32 bug here as the Win32
interfaces seem subtly different to vanilla POSIX.

The following test/break can probably be removed if Win32 needs
the same fix.
Merge pull request #45 from mutability/net-client-eof-handling

Detect client EOF properly. Handle EWOULDBLOCK.