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/rdataslab.c

@@ -16,13 +16,13 @@
 
 #include <isc/mem.h>
 #include <isc/region.h>
+#include <isc/result.h>
 #include <isc/string.h> /* Required for HP/UX (and others?) */
 #include <isc/util.h>
 
 #include <dns/rdata.h>
 #include <dns/rdataset.h>
 #include <dns/rdataslab.h>
-#include <dns/result.h>
 
 /*
  * The rdataslab structure allows iteration to occur in both load order