Solution: add a ZMQ_MSG_T_SIZE context read-only option so that
wrappers can call zmq_ctx_get (ctx, ZMQ_MSG_T_SIZE) to get the
size at runtime.

Document terminating NUL behavior in API more explicitly

This addresses #2169.

Problem: inconsistent indentation and tabs in code

Problem: multicast is spelled incorrectly in udp_address class

Solution: Fix typo of 'mutlicast' -> 'multicast'

Fix htons copy pasta

Problem: IPV6_TCLASS setsockopt fails on IPv4 socket

Solution: if setsockopt errors out and errno is set to ENOPROTOOPT
(or EINVAL on OSX) ignore it and carry on.

Make ZMQ_TOS work with IPv6 sockets

Summary:
To set `Type Of Service` for IP layer packets ZMQ provides `ZMQ_TOS` socket
option. However this only works for v4 sockets. Considering things are moving to
IPv6 heavily (especially within enterprise networks), ZMQ should support setting
`traffic class` for v6 based on `ZMQ_TOS`.

There is a subtle difference between v4 and v6 in terms of the positioning of
field but TOS has same meaning in both v4 and v6. Linux provides following APIs
for v4/v6 to set TOS field value.
```
// For v4
setsockopt(fd, IPPROTO_IP, IP_TOS, tos, sizeof(tos));

// For v6
setsockopt(fd, IPPROTO_IPV6, IPV6_TCLASS, tos, sizeof(tos));
```

Test Plan:
Make sure Cmake works fine and all tests passes.
Imported this change to `OpenR` project and tested on our local testbed.
Captured some packets exchanged between PUB/SUB and ROUTER/ROUTER sockets
pairs. Verify that `TCLASS` value is set as per expectation.

Tasks: #2208

Problem: 4.2.0 is out, time to bump version for development

Problem: linker fails looking for dladdr

Solution: search and add it via AC_CHECK_LIB when building with
libunwind, as the backtrace function uses dladdr. This problem
only appears on some distributions and with some compiler/toolchain
versions.

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

Problem: NEWS says ZMQ_BLOCKY is a sock opt, but it's a ctx opt

Solution: fix it

Problem: linking fails on glibc 2.24 with dladdr missing

Solution: try to find dladdr function on Linux - glibc systems provides
dlopen/dladdr in libdl and not in libc itseld.

Problem: curve_keygen not packaged by RPM spec

Solution: add new libzmq-tools package to ship it

Problem: some errors on Debian + kFreeBSD

Solution: do not fail on kFreeBSD if this feature is not available
at runtime.
Thanks Steven Chamberlain <steven@pyro.eu.org> for the patch!

Solution: ignore it on kFreeBSD, as it is not an error
Thanks Steven Chamberlain <steven@pyro.eu.org> for the patch!

Problem: curve_keygen not build unless --enable-curve is passed

Solution: test ENABLE_CURVE_KEYGEN and enable it only if
zmq_enable_curve_keygen=yes nad enable_curve=yes. Additionally set
enable_curve=yes for libsodium and tweetnacl, so it is enabled
implicitly and fixes the problem.

Fix small typo

Problem: NEWS says scatter/gather support UDP but they don't

Solution: fix NEWS

Problem: NEWS does not mention UDP multicast transport

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

Problem: NEWS out of date

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.

 Add WinXP compatibility