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/dns/nsec.c

@@ -14,6 +14,7 @@
 #include <stdbool.h>
 
 #include <isc/log.h>
+#include <isc/result.h>
 #include <isc/string.h>
 #include <isc/util.h>
 
@@ -24,7 +25,6 @@
 #include <dns/rdataset.h>
 #include <dns/rdatasetiter.h>
 #include <dns/rdatastruct.h>
-#include <dns/result.h>
 
 #include <dst/dst.h>