[posix] handle signals SIGHUP or SIGTERM and exit (#2930)

This commit adds signal handler in posix example platform for
signals SIGHUP and SIGTERM. Upon receiving either signal the
process is terminated using `exit(0)`.

examples/platforms/posix/platform-posix.h

@@ -70,6 +70,7 @@ __forceinline void timersub(struct timeval *a, struct timeval *b, struct timeval
 #include <fcntl.h>
 #include <netinet/in.h>
 #include <poll.h>
+#include <signal.h>
 #include <sys/select.h>
 #include <sys/socket.h>
 #include <sys/stat.h>

examples/platforms/posix/platform.c

@@ -57,9 +57,17 @@ uint32_t WELLKNOWN_NODE_ID = 34;
 
 extern bool gPlatformPseudoResetWasRequested;
 
+static volatile bool gTerminate = false;
+
 #ifndef _WIN32
 int    gArgumentsCount = 0;
 char **gArguments      = NULL;
+
+static void handleSignal(int aSignal)
+{
+    (void)aSignal;
+    gTerminate = true;
+}
 #endif
 
 void PlatformInit(int aArgCount, char *aArgVector[])
@@ -85,6 +93,9 @@ void PlatformInit(int aArgCount, char *aArgVector[])
 
     gArgumentsCount = aArgCount;
     gArguments      = aArgVector;
+
+    signal(SIGTERM, &handleSignal);
+    signal(SIGHUP, &handleSignal);
 #endif
 
     NODE_ID = (uint32_t)strtol(aArgVector[1], &endptr, 0);
@@ -149,6 +160,11 @@ void PlatformProcessDrivers(otInstance *aInstance)
         }
     }
 
+    if (gTerminate)
+    {
+        exit(0);
+    }
+
     platformUartProcess();
     platformRadioProcess(aInstance);
     platformAlarmProcess(aInstance);