ellzey/tac_plus_AFL

Recently we have had the need for tac_plus to temporarily disable user
accounts based on the number of authentication failures the user has had
in a defined window of time.

Attached is a patch against F4.0.4.15 with the previously submitted
acct+syslog patch (if this is a problem please inform me and I will
patch against the base F4.0.4.15 tree). 

The following global configuration parameter has been added:

auth-fail-lock $int1 $int2 $int3

Where $int1 is the number of authentication failures
Where $int2 is the window (in seconds) in which to watch for auth fails
Where $int3 is the number of seconds to disable the user. 

An example would be:
# Watch for 10 authentication failures within 60 seconds, if triggered
# disable user for 120 seconds.
auth-fail-lock 10 60 120

The tac_plus daemon will log when a trigger is hit, and when the account
has been re-enabled:
Jun 23 14:51:36 192.168.0.1 tac_plus[27731]: User mark has been disabled for 120 seconds
Jun 23 14:53:46 192.168.0.1 tac_plus[28244]: Re-enabling account: mark  

Unfortunately since tac_plus is a forked architecture, I had to achieve
persistence of data via IPC. I understand that some may be weary of this
mechanism so they can turn the feature off at compile time by passing
the --disable-afl flag to configure.