Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

- Added rad_demangle() for demangling user-passwords (needed for
MS-CHAPv1 MPPE-keys).
- Added rad_demangle_mppe_key() for demangling mppe-keys (needed
for MPPE-keys).
- Added some typecasts for avoiding compiler warnings.
- Fix: better handle wrong usage of the lib (if the programmer
has not called rad_create_request() but rad_put_*(), then a
weird error message was returned).
- Added a new function for putting the Message-Authenticator.
- Verify the Message-Authenticator, if it was found inside a
response packet and silently drop the packet, if the validation
failed.
- Implicitly put the Message-Authenticator, if the EAP-Message
attribute was added.
- Added some missing defines.

Submitted by: Michael Bretterklieber
PR: 46555

Add a prototype for rad_get_vendor_attr().

Forgotten by: brian

Add support for vendor specific RADIUS extensions.

Only the extensions from rfc2548 are specified for now.