Fix overloaded eq/ne comparisons (GH #116 by demerphq, GH #117 by Graham Knopp):
detect strings, protect from endless recursion. false is now ne "True".

clarify eq/ne rules in the docs:

false is eq to the empty string or the string 'false' or the special
empty string C<!!0>, i.e. C<SV_NO>, or the numbers 0 or 0.0.

true is eq to the string 'true' or to the special string C<!0>
(i.e. C<SV_YES>) or to the numbers 1 or 1.0.