fix captive portal prune sessions, radius still needs work, old code copied back but not sure if this works

LOG_AUTH is not a level, it's a facility which can't be cherry-picked
from the syslog() call.  Remove it.  Also stops spewing of auth message
into console.

Discussed with: Jos

Since this might break a few setups I'll postpone the zap
until after releasing the next snapshot on our road to 15.1.