Replace uses of the :shared attribute by calls to share()

The t/dualvar.t is supposed to run on perl 5.6, for which the :shared
attribute makes no sense and cause a compilation error. Plain calls to
threads::shared::share() achieve the same goal while keeping the code
compilable by perl 5.6.
Correct the number of tests in t/dualvar.t

One test was not counted for the skipped block on 5.6.0. This caused no
problems in greater versions of perl because the total number of tests was
also off by one.
Merge branch 'fix-dualvar-test-for-560' of git://github.com/vpit/Scalar-List-Utils into vpit-fix-dualvar-test-for-560
Fix test count again; load threads::shared at BEGIN time so share()'s prototype takes effect properly