CURVE: Implement client-side ERROR handling

Prefix error-reason with length in ERROR command

NULL: Correct ERROR command format

Finish ERROR handling for PLAIN mechanism

PLAIN: Implement ERROR handling in client

test case for zmq_getsockopt/ZMQ_IDENTITY_FD

Simplify VS project configs and harden MSBUILD script

Update cmake, mingw32 and msvc builds

This became necessary after b9c27481

This became necessary after 22d6a974

Batched VS builds, updated libsodium imports

Documentation for ZMQ_IDENTITY_FD sockopt

Update  cmake for split of plain_mechanism

Implement ZMQ_IDENTITY_FD sock option for linking socket identity to FD

Split plain_mechanism into client and server part

add untracked test_stream_timeout into .gitignore and tests/Makefile.am

Issue #1017: add ZMQ_HANDSHAKE_IVL time limit on connection handshake