Pickup: Pickup() and PickupChan() parameter parsing improvements.

* Made Pickup() and PickupChan() tollerate empty pickup values.  i.e., You
can now have Pickup(&&exten@context).

* Made PickupChan() use the standard option flag parsing code.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@402829 65c4cc65-6c06-0410-ace0-fbb531ad65f3