Add header guards for consistency in signal.h

Co-authored-by: BernardXiong <1241087+BernardXiong@users.noreply.github.com>

include/signal.h

@@ -106,21 +106,47 @@
 #define SIGUNUSED SIGSYS
 #endif
 
+#ifndef SA_NOCLDSTOP
 #define SA_NOCLDSTOP    1
+#endif
+#ifndef SA_NOCLDWAIT
 #define SA_NOCLDWAIT    2
+#endif
+#ifndef SA_SIGINFO
 #define SA_SIGINFO      4
+#endif
+#ifndef SA_ONSTACK
 #define SA_ONSTACK      0x08000000
+#endif
+#ifndef SA_RESTART
 #define SA_RESTART      0x10000000
+#endif
+#ifndef SA_NODEFER
 #define SA_NODEFER      0x40000000
+#endif
+#ifndef SA_RESETHAND
 #define SA_RESETHAND    0x80000000
+#endif
+#ifndef SA_RESTORER
 #define SA_RESTORER     0x04000000
+#endif
 
+#ifndef _NSIG
 #define _NSIG           65
+#endif
+#ifndef SI_KERNEL
 #define SI_KERNEL       128
+#endif
 
+#ifndef SIG_SETMASK
 #define SIG_SETMASK     0   /* set mask with sigprocmask() */
+#endif
+#ifndef SIG_BLOCK
 #define SIG_BLOCK       1   /* set of signals to block */
+#endif
+#ifndef SIG_UNBLOCK
 #define SIG_UNBLOCK     2   /* set of signals to, well, unblock */
+#endif
 
 #ifndef _SIG_FUNC_PTR_DEFINED
 typedef void (*_sig_func_ptr)(int);
@@ -142,26 +168,56 @@ typedef unsigned long sigset_t;
 #define SIG_ERR ((_sig_func_ptr)-1) /* Error return */
 #endif
 
+#ifndef SI_USER
 #define SI_USER         0x01    /* Signal sent by kill(). */
+#endif
+#ifndef SI_QUEUE
 #define SI_QUEUE        0x02    /* Signal sent by sigqueue(). */
+#endif
+#ifndef SI_TIMER
 #define SI_TIMER        0x03    /* Signal generated by expiration of a
                                timer set by timer_settime(). */
+#endif
+#ifndef SI_ASYNCIO
 #define SI_ASYNCIO      0x04    /* Signal generated by completion of an
                                asynchronous I/O request. */
+#endif
+#ifndef SI_MESGQ
 #define SI_MESGQ        0x05    /* Signal generated by arrival of a
                                message on an empty message queue. */
+#endif
 
+#ifndef CLD_EXITED
 #define CLD_EXITED      1
+#endif
+#ifndef CLD_KILLED
 #define CLD_KILLED      2
+#endif
+#ifndef CLD_DUMPED
 #define CLD_DUMPED      3
+#endif
+#ifndef CLD_TRAPPED
 #define CLD_TRAPPED     4
+#endif
+#ifndef CLD_STOPPED
 #define CLD_STOPPED     5
+#endif
+#ifndef CLD_CONTINUED
 #define CLD_CONTINUED   6
+#endif
 
+#ifndef SIGEV_SIGNAL
 #define SIGEV_SIGNAL    0
+#endif
+#ifndef SIGEV_NONE
 #define SIGEV_NONE      1
+#endif
+#ifndef SIGEV_THREAD
 #define SIGEV_THREAD    2
+#endif
+#ifndef SIGEV_THREAD_ID
 #define SIGEV_THREAD_ID 4
+#endif
 
 #ifndef _UNION_SIGVAL_DEFINED
 union sigval