Large patch from Frank Cusack <fcusack@fcusack.com> to add proper

support for MS-CHAP (client and server are now supported.)

Allow another plugin to select a different RADIUS server.

Modified radiusclient library to include two new APIs:
rc_acct_using_server and rc_auth_using_server in which caller specifies
which RADIUS servers to use, instead of using the default ones in the
config file.  The /etc/radiusclient/servers file must still contain
secrets for those servers.