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/keytable.c

@@ -16,6 +16,7 @@
 #include <isc/mem.h>
 #include <isc/print.h>
 #include <isc/refcount.h>
+#include <isc/result.h>
 #include <isc/rwlock.h>
 #include <isc/string.h> /* Required for HP/UX (and others?) */
 #include <isc/util.h>
@@ -28,7 +29,6 @@
 #include <dns/rdatalist.h>
 #include <dns/rdataset.h>
 #include <dns/rdatastruct.h>
-#include <dns/result.h>
 
 #define KEYTABLE_MAGIC	   ISC_MAGIC('K', 'T', 'b', 'l')
 #define VALID_KEYTABLE(kt) ISC_MAGIC_VALID(kt, KEYTABLE_MAGIC)