Fixed zmq_errno function definition
Minor whitespace fixes in man page
Updated security mechanisms to use variable-length commands

RFC23, RFC24, RFC26 now use variable-length command names that
end in null octet (valid C strings) instead of fixed-length
space padded strings.
Simplify ZMQ_STREAM socket implementation, part II
Return EINVAL when trying to use CURVE without libsodium
Merge pull request #596 from hintjens/master

Problem: mechanisms are lagging behind specs