[gdal-dev] Suggested new typemap for the bindings

[Ari Jolma]

I defined a new type VSI_RETVAL to be used instead of int with VSI 
functions that return -1 in error and set errno.

For the Perl bindings I defined two typemaps for this type. One which 
strips the return value and one which throws an error if the return 
value is -1. The error is strerror(errno).

I think this improves the usability of those VSI functions from the 
binded languages.

Ari