Solution: avoid cloning libsodium, default build is tweetnacl

Solution: checkout stable branch before building

Solution: build by default with tweetnacl, and add sub-build project
to test libsodium

Problem: tweetnacl sources are a mess

- they have no copyright / license statement
- they are in some randomish directory structure
- they are a mix of postable and non-portable files
- they do not conform to conditional compile environment

Overall, it makes it rather more work than needed, in build scripts.

Solution: clean up tweetnacl sauce.

- merged code into single tweetnacl.c and .h
- standard copyright header, DJB to AUTHORS
- moved into src/ along with all other source files
- all system and conditional compilation hidden in these files
- thus, they can be compiled and packaged in all cases
- ZMQ_USE_TWEETNACL is set when we're using built-in tweetnacl
- HAVE_LIBSODIUM is set when we're using external libsodium

It's especially annoying to see this:

  --enable-perf           Build performance measurement tools [default=yes].
  --disable-eventfd       disable eventfd [default=no]
  --enable-curve-keygen   Build curve key-generation tool [default=yes].

Solution: all options should explain the non-default case. Also
the language should be enable/disable, with/without, rather than
yes/no. E.g. '--without-docs'.

Problem: old filenames in .gitignore

Problem: autotools platform.hpp is not compatible with CMake

Solution: remove them

Specifically, the poller detection code does not set macros in
platform.hpp. The configure script passed them as -D on the command
line.

Solution: rewrite the poller detection code.

Problem: use of libsodium vs. tweetnacl is confused

This happens if you first configure with autotools, and then run
cmake. The problem is that the compiler finds the old src/platform.hpp
before looking for the one generated by CMake. Further, there are a
set of macros that configure passes via the command line, yet CMake
passes via platform.hpp. (HAVE_xxx for pollers, at least.) This means
you can't do a CMake build using the autotools platform.hpp.

Solution: remove any src/platform.hpp when running cmake. This is a
workaround. I'll fix the inconsistent macros separately.

It's unclear which we need and in the source code, conditional code
treats tweetnacl as a subclass of libsodium, which is inaccurate.

Solution: redesign the configure/cmake API for this:

* tweetnacl is present by default and cannot be enabled
* libsodium can be enabled using --with-libsodium, which replaces
  the built-in tweetnacl
* CURVE encryption can be disabled entirely using --enable-curve=no

The macros we define in platform.hpp are:

    ZMQ_HAVE_CURVE    1        //  When CURVE is enabled
    HAVE_LIBSODIUM    1        //  When we are using libsodium
    HAVE_TWEETNACL    1        //  When we're using tweetnacl (default)

As of this patch, the default build of libzmq always has CURVE
security, and always uses tweetnacl.

Cleaning up recent option names

And I'm on a reasonably sized laptop. I think allocating INT_MAX
memory is dangerous in a test case.

Solution: expose this as a context option. I've used ZMQ_MAX_MSGSZ
and documented it and implemented the API. However I don't know how
to get the parent context for a socket, so the code in zmq.cpp is
still unfinished.

These options are confusing and redundant. Their names suggest
they apply to the tcp:// transport, yet they are used for all
stream protocols. The methods zmq::set_tcp_receive_buffer and
zmq::set_tcp_send_buffer don't use these values at all, they use
ZMQ_SNDBUF and ZMQ_RCVBUF.

Solution: merge these new options into ZMQ_SNDBUF and ZMQ_RCVBUF.

This means defaulting these two options to 8192, and removing the
new options. We now have ZMQ_SNDBUF and ZMQ_RCVBUF being used both
for TCP socket control, and for input/output buffering.

Note: the default for SNDBUF and RCVBUF are otherwise 4096.

The proper name is ZMQ_THREAD_SAFE

Solution: fix in the documentation.

This option has a few issues. The name is long and clumsy. The
functonality is not smooth: one must set both this and
ZMQ_XPUB_VERBOSE at the same time, or things will break mysteriously.

Solution: rename to ZMQ_XPUB_VERBOSER and make an atomic option.

That is, implicitly does ZMQ_XPUB_VERBOSE.

Solution: rename to ZMQ_MAXRT

This is the option name used on Windows, so easier to use and
remember.

Problem: ZMQ_USEFD does not follow conventions

Solution: rename socket option (and variables and files) from usefd
to use_fd.

Problem: ZMQ_PRE_ALLOCATED_FD is too long

Solution: rename socket option (and variables and files) from
pre_allocated_fd to usefd.

CMake and tweetnacl CI

Solution: add builds/cmake/ci_build.sh and call it from travis.yml

Solution: bump CMake required version to 2.8.12 to avoid:

CMake Error at tests/CMakeLists.txt:110 (target_include_directories):
  Unknown CMake command "target_include_directories".

Solution: add builds/tweetnacl/ci_build.sh and add it in travis.yml

Solution: disable -pedantic when building with tweetnacl to avoid
warning about "long long" not existing in ISO C++ 98

fallback on tweetnacl if libsodium is not found and not explicitly requested

allows building with tweetnacl without cmake

Fixes to Windows builds

Solution: fix these.

Solution: at least for vs2015, add vs2015/build.bat to work the
same was as zproject.

Fix a typo in "Add ZMTP heartbeats" changes

Change to detect POSIX Thread priority support properly

Use memcpy instead of assuming option values are aligned