[xmlsec] loading crypto engines as plugins

[Aleksey Sanin]

The "build changes" discussion raised a question about using
crypto engines as "plugins". It is possible to do this right now but
you have to write some code by yourself. However, I think I know
a relativly simple change to xmlsec (API/ABI compatible with
current version) that can make loading crypto libraries at runtime
much easier. Please note that I still not sure that I want to go
in dlopen/LoadLibrary/... bussiness right now.

I can do it in a couple days but first I would like to ask a question:
does anyone have a need in such an improvement? Any use cases for it?

Aleksey