Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

- Rewrite radius servers traversal algorithm.
- Add functions for working with IPv6 attributes.

Approved by: ae

- 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.