[xmlsec] Problems writing multithreaded application

[Aleksey Sanin]

I believe the problem was how to pass "context" information
from the application to the crypto transforms. The proposed
solution is to store the necessary data in TLS and then access
them from the crypto transform. Obviously, you have to do it
from each thread :)

There is another option. Add "callbacks" to xmlSecDSigCtx and
xmlSecEncCtx that would be called by xmlsec after creating
transforms chain. Then application can use these calbacks to
set "userData" in the transform contexts as needed.

Aleksey