error: Refactor error reporting

Introduce xmlStrVASPrintf, trying to handle buggy snprintf
implementations.

Introduce xmlSetError to set errors atomically.

Introduce xmlUpdateError to set an error, fixing up node, file and line.

Introduce helper function xmlRaiseMemoryError.

Make legacy error handlers call xmlReportError, avoiding checks in
xmlVRaiseError.

Remove fragile support for getting file and line info from XInclude
nodes.

Author: nwellnhof