[krbdev.mit.edu #8657] SPAKE support for Windows build

[Greg Hudson via RT]

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 
Windows.

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.