dont pollute the libxmlsec/libxml2 global state with error handlers

[crewjam]

whenever a golang thread invokes startProcessingXML() we take over the libxmlsec error handler (which is process-global) and the libxml2 error handler (which is per-thread)

If other packages are using cgo and libxml2 or libxmlsec in the same process we'll possibly break 'em, which isn't nice.

Part of the right solution is to have a c function invoked from stopProcessingXML() which unregisters our libxml2 handler.

For libxmlsec, the problem is subtler because we don't know if another goroutine might be calling xmlSecErrorsSetCallback somewhere. Ugh.

[savp-RP]

Do we have any fix for this?