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: http://svn.asterisk.org/svn/asterisk/trunk@402829 f38db490-d61c-443f-a65b-d21fe96a405b