Solution: replace with `zmq_ctx_term`

Also fix whitespace (tabs instead of spaces)

Solution: update to 2016

VMCI transport allows fast communication between the Host
and a virtual machine, between virtual machines on the same host,
and within a virtual machine (like IPC).

It requires VMware to be installed on the host and Guest Additions
to be installed on a guest.

Solution: Move the macros to a private header.

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

Solution: set defaults back to infinity, and add new context
option, ZMQ_BLOCKY that the user can set to false to get a
less surprising behavior on context termination. Eg.

    zmq_ctx_set (ctx, ZMQ_BLOCKY, false);

When Curve authentication is used, libsodium opens a file
descriptor to /dev/urandom to generate random bytes. When
the ZMQ context terminates, it should ensure that file gets
closed.

environment variables.

Well, not gibberish, but 2^31 on Linux, which is useless. The code
should probably use getrlimit on Linux and other calls depending on
the system. For now I've set the ceiling at 64K.

- limit visibility of pending_connection_t
- add const qualifiers

As per suggestion by Pieter Hintjens. Also update wording in
zmq_ctx_set manual a bit.

The new options allows querying the maximum allowed number of sockets.
This is system dependent and cannot be encoded in the include file as a
preprocessor macro: for ZMQ_USE_SELECT, this depends on the FD_SETSIZE
macro at time of library compilation, not at time of include file use.

Fixes #872

Terminate context in a child process of fork() to replace file descriptors to not interfere with parent process's context