Introduce the SESSION concept

[npyl]

A SESSION (authorized session) is our concept for maintaining admin permissions for a requester even after an NSAuthenticatedTask instance has been destroyed.

This will allow cocoasudo to work authenticated even after cocoasudo exits.

[npyl]

Concept implementation: first time you call launchAuthorized it returns a session id (unique number - randomly generated.)

Next time you call launchAuthorized pass the session id. (This way, NSAuthenticatedTask will know to which session to assign the current task.)

[npyl]

Introduce a new method: -(int)launchAuthorizedWithSession:(NSASession)sessionID.
typedef NSUInteger NSASession;

NOTE: -(int)launchAuthorized returns -1 if something failed, otherwise return a sessionID.

[npyl]

Previous commits have introduced the required base for having the SESSION concept.
Now we need to implement the storing/recovery of respective SESSION handle based on a provided sessionID.