MDEV-19508: SI_KERNEL is not on all implementations

SI_USER is, however in FreeBSD there are a couple of non-kernel
user signal infomations above SI_KERNEL.

Put a fallback just in case there is nothing available.

include/my_pthread.h

@@ -197,7 +197,6 @@ static inline int my_sigwait(sigset_t *set, int *sig, int *code)
   *code= siginfo.si_code;
   return *sig < 0 ?  errno : 0;
 #else
-#define SI_KERNEL 128
   *code= 0;
   return sigwait(set, sig);
 #endif

sql/mysqld.cc

@@ -3253,7 +3253,13 @@ pthread_handler_t signal_hand(void *arg __attribute__((unused)))
       }
       break;
     case SIGHUP:
+#if defined(SI_KERNEL)
       if (!abort_loop && origin != SI_KERNEL)
+#elif defined(SI_USER)
+      if (!abort_loop && origin <= SI_USER)
+#else
+      if (!abort_loop)
+#endif
       {
         int not_used;
 	mysql_print_status();		// Print some debug info