[krbdev.mit.edu #8657] SPAKE support for Windows build
SPAKE is implemented as a dynamically loaded plugin, with one group
implemented internally and the other three using OpenSSL. As we have
no support for building dynamic plugin modules on Windows.
The ideal approach is to add build system support for creating dynamic
plugin modules (as DLLs), and support for building against OpenSSL.
This would also make it relatively easy to add PKINIT support for
A less satisfactory approach would be to move the SPAKE client module
and edwards25519 code into libkrb5, so it can be built with the
existing Windows build system, and delegate the OpenSSL groups to a
dynamically loaded module somehow.