[xml] line number information with reader API

[Denys Duchier <duchier ps uni-sb de>]

There doesn't seem to be a way to obtain line number information
through the Reader API (except when the error handler is called), yet
most functions in the interface return an integer to report status
(and therefore certain errors).  Would it be possible to add an API
function to retrieve the line number information (it doesn't matter
much whether it is absolutely accurate as long as it can help locate
problems in XML files being processed).

I'd be very grateful.

Cheers,

-- 
Dr. Denys Duchier
Équipe Calligramme
LORIA, Nancy, FRANCE