Clean up after using randombytes from libsodium

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.
Merge pull request #91 from jbreams/master

Clean up after using randombytes from libsodium
Merge branch 'master' of github.com:jimrthy/zeromq4-x