Template private assignment testcase

This is broken and needs fixing at some point.
Problem reported by Pierre-Henri Wuillemin on swig-devel mailing list.
%naturalvar feature fixes and documentation

Fix and document the naturalvar feature override behaviour - the naturalvar
feature attached to a variable name has precedence over the naturalvar
feature attached to the variable's type. The overriding was not working
when turning the feature off on the variable's name.

Fix so that any use of the naturalvar feature will override the global
setting. Previously when set globally by -naturalvar or %module(naturalvar=1),
use of the naturalvar feature was not always honoured.
Add testcase for %attributestring on shared_ptr
Remove -nonaturalvar and nonaturalvar option in %module

These didn't work, were not documented and don't seem necessary