[xml] column information in the error structure

[Aleksey Sanin <aleksey aleksey com>]

Currently, xmlError structure includes line number but not the column
number. It would be really
nice for me if column number will also be available. However, extending
a public structure is a tricky
business :) And I would like to get everyones opinion before I do a
patch. I can see the following
options:
1) Add one more int field to the end of the xmlError structure.
2) Re-use (with renaming) int1 or int2 fields (someone might use them
already thought).

Any comments, suggestions?

Thanks,
Aleksey