Added mbedtls_net_close and use it in ssl_fork_server to correctly

disassociate the client socket from the parent process and the server
socket from the child process.

library/net_sockets.c

@@ -651,6 +651,19 @@ int mbedtls_net_send( void *ctx, const unsigned char *buf, size_t len )
     return( ret );
 }
 
+/*
+ * Close the connection
+ */
+void mbedtls_net_close( mbedtls_net_context *ctx )
+{
+    if( ctx->fd == -1 )
+        return;
+
+    close( ctx->fd );
+
+    ctx->fd = -1;
+}
+
 /*
  * Gracefully close the connection
  */