[cli][posix] allow Ctrl-D to exit (#2017)

src/cli/cli.cpp

@@ -897,7 +897,7 @@ exit:
 #ifdef OPENTHREAD_EXAMPLES_POSIX
 void Interpreter::ProcessExit(int argc, char *argv[])
 {
-    exit(0);
+    exit(EXIT_SUCCESS);
     OT_UNUSED_VARIABLE(argc);
     OT_UNUSED_VARIABLE(argv);
 }

src/cli/cli_uart.cpp

@@ -107,6 +107,7 @@ void Uart::ReceiveTask(const uint8_t *aBuf, uint16_t aBufLength)
 
 #ifdef OPENTHREAD_EXAMPLES_POSIX
 
+        case 0x04: // ASCII for Ctrl-D
         case 0x03: // ASCII for Ctrl-C
             exit(EXIT_SUCCESS);
             break;