Solution: add a switch and a default to autoconf

* Add specific option to select radix tree implementation for subscriptions (defaults to ON if draft API enabled).

Problems: some tests still use hard-coded TCP ports, wrong usage of CPPFLAGS, missing NEWS, no way to force C++98 builds and CI for it

Problem: C4 process is not easily discoverable.

Solution: Add a relicensing grant.

Solution: Add 'Contributing' section to README.md

Solution: cast the unused variable

Solution: if the compiler supports it, pass C++98-compat flags.
Currently Clang supports this flag but GCC does not.

Add a new flag to enable it, as building with C++98-compat but also
with -std=gnu++11 will cause a lot of warnings due to the backward
compat ifdefs.

Add a CI job to run it and ensure we don't break compatibility.

Solution: use CXXFLAGS for compiler flags and CPPFLAGS for preprocessor
flags.

Solution: add it

Solution: do not bind to an hard-coded port, use a wildcard.

Solution: do not bind to an hard-coded port, use a wildcard.

Solution: add them to .gitignore

* Problem: radix tree needs benchmarks and improvements

Solution: add a benchmark and make suggested improvements

Problem: #3263 not mentioned in NEWS

Solution: add it

Problem: using FD_SETSIZE for optimized_fd_set_t is a pessimization

Reapply session_base_t refactoring

Solution: do not use make_pair

Windows apparently returns `WSAENOBUFS` from `recv`

Problem: Assert being triggered when Windows returns `WSAENOBUFS` from `recv` call on large buffers.

Solution: Expect `WSAENOBUFS` as a valid response.  See https://github.com/zeromq/libzmq/issues/3263

Solution: added relicense agreement

Solution: extract functions for each protocol

Problem: build broken with Sun Studio, tests fail on debian/kfreebsd and debian/hurd, DRAFT zmq_poll is slow

Solution: remove it

Solution: use it only if there is at least one thread-safe socket,
which is not compatible with the older zmq_poll implementation.

Solution: define UNITY_WEAK_PRAGMA when building with the Sun compiler
as it does not support the weak attribute.

Solution: skip tests that use ipc://* as it's impossible to get the
address back for the client

Solution: temporary revert refactoring until it can be properly fixed.

Revert "Problem: complexity of start_connecting"

This reverts commit 6d7aeb05.

Solution: use a different system variable for debian/hurd and
debian/kfreebsd so that the tests are skipped only on hurd.

Solution: fix it

Problem: stale bot didn't parse labels correctly

Solution: quote the labels with a space, upstream documentation
(not displayed on the Github page) suggests this should finally work

Problem: stale issues linger on Github

Solution: enable Stable Bot to automatically mark an issue as stale
after 365 days of inactivity and close it after further 56 days.

Issues marked with the following labels are excluded:

 - Help Request
 - Feature Request
 - Problem reproduced
 - Critical

Problem: NEWS still out of date

Solution: mention all relevant bug fixes, features and changes

Problems: wrong version and outdated NEWS

Some changes for CMakeLists.txt: