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.

lib/ns/client.c

@@ -50,6 +50,7 @@
 #include <dns/rdatalist.h>
 #include <dns/rdataset.h>
 #include <dns/resolver.h>
+#include <dns/result.h>
 #include <dns/stats.h>
 #include <dns/tsig.h>
 #include <dns/view.h>