Generate and verify message MACs in libkrad

Generate and verify message MACs in libkrad

Implement some of the measures specified in
draft-ietf-radext-deprecating-radius-03 for mitigating the BlastRADIUS
attack (CVE-2024-3596):

* Include a Message-Authenticator MAC as the first attribute when
  generating a packet of type Access-Request, Access-Reject,
  Access-Accept, or Access-Challenge (sections 5.2.1 and 5.2.4), if
  the secret is non-empty.  (An empty secret indicates the use of Unix
  domain socket transport.)

* Validate the Message-Authenticator MAC in received packets, if
  present.

FreeRADIUS enforces Message-Authenticator as of versions 3.2.5 and
3.0.27.  libkrad must generate Message-Authenticator attributes in
order to remain compatible with these implementations.

[ghudson@mit.edu: adjusted style and naming; simplified some
functions; edited commit message]

ticket: 9142 (new)
tags: pullup
target_version: 1.21-next