Creating a new pre-authentication plugin

[Danilo Almeida]

<quote from="Alexandro">
I we had the table of valid handlers, we could add a expiration time.
>From time to time, mechglue can lookup for expired contexts and delete
them. Though I think this may be highly inefficient.
<quote>

Alexandro,

Note that you can just link these into a expiration linked list as follows:

1) When adding an entry, add it to the back.

2) When checking for expiration, look at the front of the list removing
   Entries until you see an expiration time greater than now.

You can have #2 happen whenever you are adding to the list or any other opportune time when the plugin is getting called.

- Danilo