Exposing functions related to CurveZMQ authentication.

[dentonbriandavid]

This pull request exposes several additional functions in a more recent version of ZeroMQ (specifically 4.2.2).

Additionally, the user can specify the number of io_threads to use for server and client sockets.

I have not created new documentation for the additional functions so the following warning appears in R CMD check:

Undocumented code objects:
‘close.socket’ ‘get.curve.server’ ‘get.io_threads’ ‘get.keypair’
‘get.public.key’ ‘get.secret.key’ ‘get.server.key’ ‘set.curve.server’
‘set.public.key’ ‘set.secret.key’ ‘set.server.key’

A warning will also appear in R CMD check because the function signature of close.socket does not follow the signature of the S3 generic base::close.

[jeroen]

This breaks the installation on most distributions that ship with an older version of libzmq. Can you please #if new stuff conditional on the version of libzmq and raise an error if something is called which requires a more recent version of libzmq than that is available?

Here is an example of a similar trick in the magick package.

[jeroen]

I think you forgot to remove some debugging? It complains that the rzmq2 package is missing.