check_radius: Support FreeRADIUS Client library

Allow for using the FreeRADIUS Client library instead of radiusclient or
radiusclient-ng.  The latter two projects are dead.

Closes #1231.
configure.ac: Change RADIUS library preferences

Prefer the FreeRADIUS Client library over radiusclient-ng, and prefer
that one over the original radiusclient library.
check_radius.c: Add newline to die() calls

Our die() function doesn't append a newline character to the message.