Took me over 8 hours to track down the cause of bizarre link
errors when building with libsodium. The VS project files are
not simple things.

Note to self and other maintainers: when someone is obviously
out of their depth, do not merge their changes to build scripts
without cynical appraisal.

Solution: undo the damage.

Caused by error in last commit which used HAVE_LIBSODIUM instead
of HAVE_SODIUM.

Solution: use HAVE_LIBSODIUM as we do in other configure scripts.

The project is called 'libsodium' and not 'sodium'.

There is an option to enable/disable libsodium via the Visual Studio
UI. This is not practical for command-line usage (via msbuild).

Solution: add configure.bat that searches for libsodium in sibling
directory to libzmq; if it finds it, defines HAVE_LIBSODIUM 1.

This is consistent with zproject, which has the same problem and
is getting the same solution for all dependency resolution on
Windows.

Note that this approach also provides a way to support tweetnacl
via a configure option.

Also, removed duplicate props files and re-copy in configure.bat
as it was an insane nightmare to update these by hand. (and not
clear that they were identical. Now it's forced.)

Leading to extra work when copying between versions of VS.

Solution: remove second unnecessary definition.

Solution: add missing project files

(Also, updated FD_SETSIZE to 16K as a bonus.)

Solution: fixed project files, and warning in timers.cpp

- zmq_utils.h was removed
- udp_address.cpp and udp_engine.cpp were added

Solution: patch all affected build scripts

I hope restoring this from a previous version doesn't do any damage, it
is missing in the current version of this repository and is vital for
the lib to compile anywhere.

Altered VS2015 files to enable building of the library.

Solution: add these new classes to MSVC project files

Solution: fix and try again.

(We could start to think of using zproject to generate these build
scripts as the only difference is C/C++)

Solution: fix it.

Solution: add these

Solution: add to project dependencies