Simplify way we tag unreachable code with only ISC_UNREACHABLE()

Previously, the unreachable code paths would have to be tagged with:

    INSIST(0);
    ISC_UNREACHABLE();

There was also older parts of the code that used comment annotation:

    /* NOTREACHED */

Unify the handling of unreachable code paths to just use:

    UNREACHABLE();

The UNREACHABLE() macro now asserts when reached and also uses
__builtin_unreachable(); when such builtin is available in the compiler.

lib/isc/interfaceiter.c

@@ -125,8 +125,7 @@ get_addr(unsigned int family, isc_netaddr_t *dst, struct sockaddr *src,
 		}
 		break;
 	default:
-		INSIST(0);
-		ISC_UNREACHABLE();
+		UNREACHABLE();
 	}
 }