client: Create agent data structure and pass it in function callbacks

client: Create agent data structure and pass it in function callbacks

Collect the agent variables into a data structure and update the agent
functionality to pass this data structure around as function callback
user data.

Update the agent mode input functionality to store both the callback
function and user data pointers. Notice that only only one input
callback can be handled at any one time due to the input handling itself
and the simple storing of callback and user data pointers.