Update  cmake for split of plain_mechanism
Merge pull request #1025 from ricnewton/master

Update  cmake for split of plain_mechanism
Merge pull request #1026 from sdrsdr/identity_fd

Documentation for ZMQ_IDENTITY_FD sockopt
Updated libsodium imports.
Ignore error on concurrent file copy in parallel VS builds.
Batched VS builds.
Merge pull request #1027 from evoskuil/master

Batched VS builds, updated libsodium imports
Update build files after plain_mechanism split

This became necessary after 22d6a9740
Update build files to include metadata

This became necessary after b9c274814
Merge pull request #1029 from olafmandel/master

Update cmake, mingw32 and msvc builds
Harden MSBUILD base script against various VS installations.
Optimize VS projects and common props.
Merge pull request #1030 from evoskuil/master

Simplify VS project configs and harden MSBUILD script
Merge branch 'master' of https://github.com/zeromq/libzmq into identity_fd