Set HWM a bit higher than 1M (message got lost)

On my machine messages got lost with HWM of exactly 1M.

According to documentation of zmq_setsockopt the actual limit might be much
smaller than the provided value. Is there a possibility to find out how much
exactly?
Merge pull request #337 from sercxanto/master

Need to increase ZMQ_SNDHWM a bit more in syncpub example