1. stl container is not thread safety
2. rm_fd() and loop() end to clear the retired event source will in multi-thread operation
3. may be crashed in reaper thread to delete the items in the std::vector as the source is nullptr

Problem: PR #2236 breaks the build

Solution: Revert "avoid crashing in the multi-thread operation for std::vector"

This reverts commit e1368bda.

avoid crashing in the multi-thread operation for std::vector

1. stl container is not thread safety
2. rm_fd() and loop() end to clear the retired event source will in multi-thread operation
3. may be crashed in reaper thread to delete the items in the std::vector as the source is nullptr

Problem: CMake build uses library version as the ABI version

Solution: set SOVERSION in CMakeLists to match the SONAME generated
by libtool so that there is no mismatch between the output of the
*NIX build systems.

Before:
$ ls -l
total 2696
lrwxrwxrwx 1 luca luca      15 Dec  1 22:36 libzmq.so -> libzmq.so.4.2.0
lrwxrwxrwx 1 luca luca      15 Dec  1 22:36 libzmq.so.4.2.0 -> libzmq.so.4.2.1
-rwxr-xr-x 1 luca luca  906168 Dec  1 22:36 libzmq.so.4.2.1
$ readelf -d libzmq.so.4.2.1 | grep SONAME
 0x000000000000000e (SONAME)    Library soname: [libzmq.so.4.2.0]

After:
$ ls -l
total 2700
lrwxrwxrwx 1 luca luca      15 Dec  1 22:31 libzmq.so -> libzmq.so.5.1.0
-rwxr-xr-x 1 luca luca  906168 Dec  1 22:31 libzmq.so.4.2.1
lrwxrwxrwx 1 luca luca      15 Dec  1 22:31 libzmq.so.5.1.0 -> libzmq.so.4.2.1
$ readelf -d libzmq.so.4.2.1 | grep SONAME
 0x000000000000000e (SONAME)    Library soname: [libzmq.so.5.1.0]

* bug fix: AIX pollset error

buf fix:  AIX only, pollset 'rm_fd' set fd_table to null first then set pe->fd to retired_fd

* remove extra spaces

remove extra spaces

Problem: Windows does not support IPV6_TCLASS

Solution: don't use it on Windows.
There is a Windows-specific API for ToS, even IPv4 does not work and
it's just a silent no-op on that platform.

Problem: select broken

Solution: initialise class variables in the same order they are
declared

Solution: fix acinclude.m4 snippet that checks if select is
available to stop it erroring out.

Add a single sentence explaining what a context is

only compare pollitem.fd if pollitem.socket is NULL

item.fd should be ignored if item.socket is defined.

Failing to check .socket could cause false-matches, raising EINVAL in zmq_poller_modify_fd.

Problem: size of zmq_msg_t is not known to FFI wrappers

Solution: mention it in zmq_ctx_get manpage

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

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