net/libzmq4: Update to 4.2.3

Changelog:
https://github.com/zeromq/libzmq/releases/tag/v4.2.3

Additional port changes:

• DISTVERSION
• Added LIBUNWIND option to use devel/libunwind
• Disabled static library
• Minor formatting changes

All tests pass.

This release includes some FreeBSD specific fixes like
https://github.com/zeromq/libzmq/pull/2514/files.

PR: 224495
Submitted by: myself
Approved by: timeout expired, tcberner (mentor, implicit)