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Problem: NEWS out of date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

#define ZMQ_HAVE_WINDOWS_TARGET_XP  disable uncompatible WinAPI

1. Disable call if_indextoname()
2. Emulate windows Condition Variable API in class condition_variable_t with std::condition_variable
 	
This code can be compiled in MSVC 2015 with option "Platform toolset: Visual Studio 2015 - Windows XP (v140_xp)"

Problem: socket option marked as draft for 4.2

Solution: move comment further below to declare new socket
options as stable for the 4.2.0 release.

Problems: need to bump NEWS and libtool version

Solution: bump ABI version fromo 5:0:0 to 6:0:1 since 4.2 is backward
compatible with 4.1, but new symbols are there

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

Problem: CMake build rejects to build with openpgm enabled

Solution: add optional lookup for openpgm via pkg-config

Suppress warning on uninitialized variable use by initializing variable.

Remove an unnecessary while (true)

This block of code will either return -1 or fall through.
The while (true) does nothing.
The braces limit the scope of int rc.