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 #1174 from jbreams/master

Clean up after using randombytes from libsodium
Merge pull request #1173 from evoskuil/master

Fix configure warning.