Problem: 4.2.1 is out, time to bump versions

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

Problems: test_monitor failing, DRAFT events leaking when draft APIs are disabled

Solution: check that the expected handshake success or failure events
are triggered in the curve security unit test

Solution: fix it

Solution: add an entry

Solution: add them to the event monitor manpage

Solution: add a note pointing out that when new events are added the
ZMQ_EVENT_ALL mask will start returning them, so applications that
can't handle that transparently should not use it and instead pick
only the event types they want.
Creating a new event type and having monitors with ZMQ_EVENT_ALL
return them will not be considered a backward incompatible change
even though the flow of events will change.

Solution: wrap the event triggering in the DRAFT ifdef as well as the
defines. This ensures that the event are returned only if the
library was built with DRAFTs enabled.

Also update the test case to expect the new events since it uses
the catch-all mask. Although the sequence of event is different and
this might be considered as an API breakage, using the catch-all
ZMQ_EVENT_ALL mask implies that, well, all events are monitored so
it's normal that new ones will be added.
Users that don't want this kind of behaviour to change can simply
monitor only the event that they care about.

Code formatting + reverted hard error handshake fail

 - Moved new events in draft section + added to zmq_draft.h
 - Removed the remainning tabs
 - Reverted the hard error (back to soft error) in curve_server.cpp

=> The feature doesn't works anymore

Fixed issue #2227

Added two new monitoring events:
 - ZMQ_EVENT_HANDSHAKE_SUCCEED is raised once the encryption handshake succeed
 - ZMQ_EVENT_HANDSHAKE_FAILED is raised when it failed
Both events are raised on server and client side.

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

Problem: debian build fails with automake < 1.14

Solution: create config subdir as a workaround if building the
packages with automake < 1.14

Solution: rework the bash test to avoid build failures as intended
and make it an easier one-liner

Solution: install asciidoc and xmlto in the libsodium build run, since
it already refreshes APT to download other packages, so the delay is
smaller

Solution: use packages on Ubuntu and brews on OSX. The packages and
the brews are always kept up to date, so it's no use to rebuild the
libsodium stable branch manually everytime.

Solution: Revert "Problem: Travis CI OSX builds are broken"

This reverts commit 531b3bf7.

Solution: narrow down the matrix to install valgrind and lcov only
for the test runs that use them.
CMake is part of the default image so no need to install it at all.

Solution: add it to the list

Solution: if available, call socketpair with SOCK_CLOEXEC flag to
make the process close the socket on fork+exec

Solution: if available, use eventfd with EFD_CLOEXEC flag to make
the process close the socket on fork+exec

Solution: if available, use epoll_create1 with EPOLL_CLOEXEC flag to
make the process close the socket on fork+exec

Solution: add paragraphs to zmq_get/setsockopt man pages

Solution: add manpage

Solution: add a comment as it's easy to miss and very tempting to
"optimize"

Mingw32 Compile

Problem: Duplicated code