Make improvements

- Clean up sigaction() code
- Add a port scanner example
- Introduce a ParseCidr() API
- Clean up our futex abstraction code
- Fix a harmless integer overflow in ParseIp()
- Use kernel semaphores on NetBSD to make threads much faster

libc/integral/c.inc

@@ -608,10 +608,10 @@ typedef struct {
 #endif
 #endif
 
-#define notpossible    \
-  do {                 \
-    asm("ud2\n\tnop"); \
-    unreachable;       \
+#define notpossible           \
+  do {                        \
+    asm("nop\n\tud2\n\tnop"); \
+    unreachable;              \
   } while (0)
 
 #define donothing \