Posix: add '#pragma GCC poison exit'

Generally exit() should not be used on Posix, because it exits the whole
program instead of only the task.

platforms/posix/src/px4_daemon/client.cpp

@@ -330,7 +330,7 @@ Client::_sig_handler(int sig_num)
 
 	if (_client_send_pipe_fd < 0) {
 		PX4_ERR("pipe open fail");
-		exit(-1);
+		system_exit(-1);
 	}
 
 	int bytes_to_send = get_client_send_packet_length(&packet);
@@ -338,7 +338,7 @@ Client::_sig_handler(int sig_num)
 
 	if (bytes_sent != bytes_to_send) {
 		PX4_ERR("write fail");
-		exit(-1);
+		system_exit(-1);
 	}
 }
 

platforms/posix/src/px4_layer/px4_posix_tasks.cpp

@@ -115,7 +115,7 @@ void
 px4_systemreset(bool to_bootloader)
 {
 	PX4_WARN("Called px4_system_reset");
-	exit(0);
+	system_exit(0);
 }
 
 px4_task_t px4_task_spawn_cmd(const char *name, int scheduler, int priority, int stack_size, px4_main_t entry,