Add md5 (for HTTP Digest Auth)

Some compilers complain about typedef redefinition:

```
warning: redefinition of typedef 'foo' is a C11 feature [-Wtypedef-redefinition]
```

, it would be nice to have our code warning free.

Anyway I guess we should make this module disableable for size reasons, and have the auth part of http conditionally enabled on either this or the builtin md5 impl.