Make CNSocket::kill() idempotent

CNSocket::kill() will now no longer call close() on already closed sockets.

close() should never be called on already closed file descriptors, yet
CNSocket::kill() was lacking any protection against that, despite its
use as both a high-level way of killing player connections and as a
means of ensuring that closing connections have been properly terminated
in the poll() loop.

This was causing close() to be erroneously called on each socket at least
one extra time. It was also introducing a race condition where the login
and shard threads could close each other's newly opened sockets due to
file descriptor reuse when a connection was accept()ed after the first
call to close(), but before the second one. See the close(2) manpage for
details.

src/core/CNProtocol.cpp

@@ -109,7 +109,11 @@ bool CNSocket::isAlive() {
 }
 
 void CNSocket::kill() {
+    if (!alive)
+        return;
+
     alive = false;
+
 #ifdef _WIN32
     shutdown(sock, SD_BOTH);
     closesocket(sock);