SSL.Cipher: Added KeyExchangeKRB (experimental).

This is a tentative implementation of the RFC 2712
Kerberos-based key exchange.

No interoperation tests have been performed, and the
corresponding suites are not enabled.
SANE: Fixed all warnings.
Build: Added configure option --enable-werror.

The configure option --enable-werror now adds -Werror
to the set of WARN flags.