Solultion: clarify that notifications for unique subs/unsubs are always
passed, and these option only affect the behaviour for duplicated ones.

Solution: bump changelog, ABI revision, library and packaging version

Solution: add it

Solution: add it

Solution: add it

Solution: add --disable-Werror flag like the existing
--disable-pedantic or CMake's -DLIBZMQ_WERROR=OFF
Fixes #2818

Solution: check for availability in CMake and autoconf before using it

Solution: add main features and bug fixes

Solution: bump changelog, ABI revision, library and packaging version

Solution: mention main changes and bug fixes since 4.2.1

Solution: add new changelog entry, bump library version in zmq.h and
bump ABI version in configure.ac and CMakeLists.txt

Finalize changelog and bump ABI revision

Solution: fix it

Solution: add an entry

Solution: add most relevant bug fixes since 4.2.0

Solution: move it there, add it to src/zmq_draft.h and clarify in the
documentation and in the NEWS file that this option is not yet stable

Solution: mention it as it's a new public API (still draft until release)

Solution: update NEWS, include/zmq.h and packaging

Solution: fix it

Solution: fix NEWS

Solution: add mention and point to doc/zmq_udp.txt

Solution: add an overview of the new socket options, new APIs,
DRAFT mechanism (and DRAFT APIs). And a dedication.

Solution: add release notes from 4.0.x and 4.1.x releases to
capture all important solved bugs.

Solution: add one, empty for now, fill in after RC release

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>