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

tool/build/lib/syscall.c

@@ -24,7 +24,6 @@
 #include "libc/calls/struct/iovec.h"
 #include "libc/calls/struct/rlimit.h"
 #include "libc/calls/struct/rusage.h"
-#include "libc/calls/struct/sigaction-linux.internal.h"
 #include "libc/calls/struct/sigaction.h"
 #include "libc/calls/struct/stat.h"
 #include "libc/calls/struct/sysinfo.h"

tool/net/echo.c

@@ -29,6 +29,7 @@
 #include "libc/sysv/consts/ipproto.h"
 #include "libc/sysv/consts/sock.h"
 #include "net/http/http.h"
+#include "net/http/ip.h"
 
 /**
  * @fileoverview tcp/udp echo servers/clients