[O] Honour existing restrictions if possible when visiting tasks from the agenda

[Bernt Hansen]

The following short patch series fixes org file restriction
handling for me.  I'm regularly narrowing to subtrees for a
project from the agenda and then using the agenda to visit tasks
in the subtree.  Each time I visit a task, clock in, or
regenerate the agenda my restriction is removed in the org file.
The following patch series fixes this behaviour.