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/tsig.c

@@ -19,6 +19,7 @@
 #include <isc/mem.h>
 #include <isc/print.h>
 #include <isc/refcount.h>
+#include <isc/result.h>
 #include <isc/serial.h>
 #include <isc/string.h> /* Required for HP/UX (and others?) */
 #include <isc/time.h>
@@ -33,11 +34,8 @@
 #include <dns/rdatalist.h>
 #include <dns/rdataset.h>
 #include <dns/rdatastruct.h>
-#include <dns/result.h>
 #include <dns/tsig.h>
 
-#include <dst/result.h>
-
 #include "tsig_p.h"
 
 #define TSIG_MAGIC	  ISC_MAGIC('T', 'S', 'I', 'G')