Fix error codes passed to connection callbacks

Fix error codes passed to connection callbacks

Commit 9ee60e7a17bf34c7ef7f4d79e6a00ca45444ec8c erroneously introduced
duplicate conditions to several existing conditional statements
responsible for determining error codes passed to connection callbacks
upon failure.  Fix the affected expressions to ensure connection
callbacks are invoked with:

  - the ISC_R_SHUTTINGDOWN error code when a global netmgr shutdown is
    in progress,

  - the ISC_R_CANCELED error code when a specific operation has been
    canceled.

This does not fix any known bugs, it only adjusts the changes introduced
by commit 9ee60e7a17bf34c7ef7f4d79e6a00ca45444ec8c so that they match
its original intent.