Removes duplicated declaration
Problem: selftests fail when libzmq is built w/o libsodium

When CZMQ and libzmq disagree about libsodium support, it shows
as a hard-to-diagnose assertion in the zsockopt selftest.

Solution: explicitly check for this error and report it at the
start of CZMQ selftesting.

Fixes #494

(Also improved zsys_test coverage.)