Make isc_result a static enum

Remove the dynamic registration of result codes.  Convert isc_result_t
from unsigned + #defines into 32-bit enum type in grand unified
<isc/result.h> header.  Keep the existing values of the result codes
even at the expense of the description and identifier tables being
unnecessary large.

Additionally, add couple of:

    switch (result) {
    [...]
    default:
        break;
    }

statements where compiler now complains about missing enum values in the
switch statement.

bin/tests/system/hooks/driver/test-async.c

@@ -27,8 +27,6 @@
 #include <isc/types.h>
 #include <isc/util.h>
 
-#include <dns/result.h>
-
 #include <ns/client.h>
 #include <ns/events.h>
 #include <ns/hooks.h>