http.auth: Add new function to verify passwords

This new function supports two schemes for passwords:
- Old-style cleartext passwords
- Hashed passwords according to RFC2617 (H(A1))

Schemes are differentiated by their prefix, a concept also
used in OpenLDAP. Cleartext passwords can no longer start
with an opening brace ("{") unless they're prefixed with
"{cleartext}" (case insensitive).

Currently there's no documentation for rapi_users at all.
It'll be in a consecutive patch.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>