Add the following functions:

rad_request_authenticator()
  Returns the Request-Authenticator relevant to the most recently received
  RADIUS response.

rad_server_secret()
  Returns the Shared Secret relevant to the most recently received
  RADIUS response.

Neither of these functions should be necessary, however, the
MS-MPPE-Recv-Key and MS-MPPE-Send-Key Microsoft Vendor Specific
attributes are supplied in a mangled (encrypted) format, requiring
this information to demangle.

It's not clear whether these functions should be replaced with a
rad_demangle() function or whether these attributes are one-offs.

Sponsored by: Monzoon