zoom functions should zoom to the min/max sacle set

[assefa]

zoom function should zoom to the min/max scale set in the map file instead of 
aboring if the zoom goes outside the scales.

[assefa]

Enhamcement : will be fixed for 4.1.

[assefa]

Still a valid point after 3 years. but I do bot want to change the behaviour 
before a release. Should still be considered for FUTURE refernece.