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Problem: NEWS out of date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.

Increment metadata refcount while it's in pending_metadata

Problem: MS VC++ build broken

Solution: try to detect architecture if building with VC++ and
hardcode pointer size accordingly.
Expressions are not allowed inside declspec intrinsics, which
includes other intrinsics.

Problem: pointer union for zmq_msg_t is a hack

Solution: use compiler's alignment attributes instead which is
clearer and less of a hack.
Pointer alignment violations causing crashes on architectures
such as sparc64 and aarch64.
This also avoid triggering ABI checkers as the change is compatible
even though applications that suffer from the bug should rebuild to
take advantage of the fix.