support for disabling cryptographic algorithms deemed insecure

[dun]

Add support for disabling cryptographic algorithms deemed insecure in order to protect against downgrade attacks. This would allow sites to selectively disable algorithms having a low security margin, or quickly respond to new cryptographic attacks against such an algorithm. This option should be set in the configuration file (#58).

While an application could use munge_ctx_set() to override the default algorithms used to encode a credential, most probably don't. Likewise, an application could, in theory, reject a decoded credential that used a particular algorithm (queried via munge_ctx_get()), but in practice, that's unlikely to happen.

The impetus for this feature is the recent announcement that SHA-1 is a Shambles. While the HMAC construct protects against such length extension attacks, attacks always get better.