Problem: was passing wrong size to zmq_getsockopt on keys

libzmq requires receiving buffer to be exactly 32, or 41 bytes
for a binary or Z85 key respectively. CZMQ was passing a 255
byte buffer, and then ignoring the EINVAL return code. This was
caught by the new --with-militant assert checking (yay!)

Solution: pass a 41 byte buffer as required by the zmq_getsockopt
man page.
Problem: did not build until CZMQ was already installed

The generated zgossip_msg class assumed czmq.h was in /usr/local/include
which is correct for all applications except CZMQ itself. Zproto now
checks for czmq.h in ../include, and uses that if possible, else uses
the file in /usr/local/include/czmq.h.
Updated man pages
Regenerated zgossip server engine
Merge pull request #528 from hintjens/master

Problem: was passing wrong size to zmq_getsockopt on keys