Add support for freeradius-client

[puleglot]

radiusclient-ng (mentioned in docs as radiusclient 0.4.0 and above) is no longer maintained upstream. It was superseded by freeradius-client. The API is mostly compatible. Header name has changed and the library name has changed.

Tested, works fine.

Probably some changes in documentation and comments in EDITME are needed, but unfortunately I'm a bad english writer.

[puleglot]

Note that exim also works with radiusclient-ng 0.5.x which is still shipped by most linux distributions (along with freeradius-client). In radiusclient-ng 0.5.0 library and header were renamed to libradiusclient-ng and radiusclient-ng.h, but I see not reason to invent new RADIUS_LIB_TYPE to support it. :)