Emit a ISC_R_CANCELED result instead of ISC_R_SHUTTINGDOWN

Emit a ISC_R_CANCELED result instead of ISC_R_SHUTTINGDOWN

When request manager shuts down, it also shuts down all its ongoing
requests. Currently it calls their callback functions with a
ISC_R_SHUTTINGDOWN result code for the request. Since a request
manager can shutdown not only during named shutdown but also during
named reconfiguration, instead of sending ISC_R_SHUTTINGDOWN result
code send a ISC_R_CANCELED code to avoid confusion and errors with
the expectation that a ISC_R_SHUTTINGDOWN result code can only be
received during actual shutdown of named.

All the callback functions which are passed to either the
dns_request_create() or the dns_request_createraw() functions have
been analyzed to confirm that they can process both the
ISC_R_SHUTTINGDOWN and ISC_R_CANCELED result codes. Changes were
made where it was necessary.