basic/capability-util: let cap_last_cap() return unsigned integer

We never return anything higher than 63, so using "long unsigned"
as the type only confused the reader. (We can still use "long unsigned"
and safe_atolu() to parse the kernel file.)

src/test/test-capability.c

@@ -243,7 +243,7 @@ static void test_ensure_cap_64bit(void) {
         assert_se(p <= 63);
 
         /* Also check for the header definition */
-        assert_se(CAP_LAST_CAP <= 63);
+        assert_cc(CAP_LAST_CAP <= 63);
 }
 
 int main(int argc, char *argv[]) {