libsodium calls abort() when /dev/urandom can't be found
even if one creates ZeroMQ context before calling chroot()[1].

This happens because crypto gets initialized on handshake,
and at that moment the process is already chroot'ed.

Solution: initialize cryptographic libraries in ctx

randombytes_close() is already there in the destructor.

[1] https://download.libsodium.org/doc/usage/index.html

Libsodium has started returning -1 in some cases.

Solution: allow and handle error returns from these calls.

Fixes #1831

Solution: Phase I - make precompiled.hpp be first file included in every source file

- 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 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.

Solution: update to 2016

This commit adds ZMTP connection heartbeats described in
http://rfc.zeromq.org/spec:37/ZMTP.

Of course people still "can" distributed the sources under the
LGPLv3. However we provide COPYING.LESSER with additional grants.

Solution: specify these grants in the header of each source file.

Solution: update for 2015

Fixes #1320

There are two todo comments in curve_client.cpp and curve_server.cpp that suggest
checking the return code of sodium_init() call. sodium_init() returns -1 on error,
0 on success and 1 if it has been called before and is already initalized:
https://github.com/jedisct1/libsodium/blob/master/src/libsodium/sodium/core.c

Solution: ensure message short nonces are strictly increasing and validate them

- also prioritize ZAP metadata over ZMTP metadata in case application
  uses same names.

* The INITIATE command vouch box is Box[C',S](C->S') instead of Box[C'](C->S),
  as recommended by https://codesinchaos.wordpress.com/2012/09/09/curvecp-1/,
  to reduce the risk of client impersonation.

* Mirrors the change in libcurve and CurveZMQ specifications.

This reverts commit bfd472f9.

* Command names changed from null terminated to length-specified
* Command frames use the correct flag (bit 2)
* test_stream acts as test case for command frames
* Some code cleanups

RFC23, RFC24, RFC26 now use variable-length command names that
end in null octet (valid C strings) instead of fixed-length
space padded strings.

- ZMQ_CURVE_PUBLICKEY for clients and servers
- ZMQ_CURVE_SECRETKEY for clients
- ZMQ_CURVE_SERVERKEY for clients
- ZMQ_CURVE_SERVER for servers
- added tools/curve_keygen.c as example
- updated man pages