Check for curve capability at runtime to skip tests if needed.

Use zmq_has() to skip test that require curve capability if
this functionality wasn't built with libsodium.

This has the effect of not causing the test suite to fail in case
we didnt build libzmq with --with-libsodium.
Disable test_auth against libzmq < 4

This should allow the test suite to run correctly against
libzmq 3.2.