Drop SYN packets during HAProxy reload

During HAProxy reload TCP connections can be reset, due to a short
window when the file descriptors are handed over to the new process.
The patch works around most of this problem by dropping SYN packets
during reload, which will lead to clients resending the packets until
the new process is started.

See discussion here:
http://www.mail-archive.com/haproxy@formilux.org/msg06885.html

We have tested this in production for weeks now, enjoying lower error rates due to less dropped connections.