clean up function manager_show_dialplan_helper()

clean up function manager_show_dialplan_helper()
reducing indentation and normalizing loops.

While doing this, remove some unused variables,
fix an uninitialized string (idaction), and mark
some places where the behaviour is not what we would expect
(e.g. an empty context is reported as an error same as
a non-existent one). Given that this function is
not in 1.4, the above can be changed without too many
backward compatibility concerns.

Not applicable to 1.4 or below.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@48458 65c4cc65-6c06-0410-ace0-fbb531ad65f3