Fix npppd not to access freed memory when it is exiting.
Fix a double free bug in parsing the config.  It had occurred when a
syntax error exists in radius {authentication,accounting}-server
configuration.  No need to free curr_radconf because it is a part of
auth_conf.

Pointed out Vladimir Seleznev
sync
get rid of mfa leftovers
Match the current state of the code.
Do not suggest using BUF_strdup() preferably to strdup() for fallacious
reasons.