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/time.c

@@ -18,12 +18,12 @@
 
 #include <isc/print.h>
 #include <isc/region.h>
+#include <isc/result.h>
 #include <isc/serial.h>
 #include <isc/stdtime.h>
 #include <isc/string.h> /* Required for HP/UX (and others?) */
 #include <isc/util.h>
 
-#include <dns/result.h>
 #include <dns/time.h>
 
 static const int days[12] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };