Merge pull request #551 from hintjens/master

zmq_msg_close clarification in man pages
Implement ZMTP/3.0 NULL mechanism
Update test_raw_sock to work with ZMTP/3.0
Merge pull request #552 from hurtonm/master

Implement ZMTP/3.0 NULL mechanism
Added configure check for libsodium
Fixed packaging for Windows - was missing errno sources