Simulator: Added remote host option (#15443)

* Added an option to the Simulator module to connect to remote Gazebo servers.

This is usefull when the Gazebo simulation is running on a different host than the PX4 instance.
For example, we are running instances of PX4 with a companion application in multiple Dockers, for swarming simulations, which then connect to a remote Gazebo server.

A "-t" input argument has been added and can be called from the rcS startup script as: simulator start -t "192.168.178.122" $simulator_tcp_port

* _tcp_remote_ipaddr defaults to nullptr. This way testing against nullptr it can be determined if localhost or remote host is desired by the user.
* Documented the option in usage().

Signed-off-by: Peter Blom <peterblom.mail@gmail.com>

src/modules/simulator/simulator_mavlink.cpp

@@ -51,6 +51,7 @@
 #include <pthread.h>
 #include <sys/socket.h>
 #include <termios.h>
+#include <arpa/inet.h>
 
 #include <limits>
 
@@ -673,6 +674,10 @@ void Simulator::run()
 	_myaddr.sin_addr.s_addr = htonl(INADDR_ANY);
 	_myaddr.sin_port = htons(_port);
 
+	if (_tcp_remote_ipaddr != nullptr) {
+		_myaddr.sin_addr.s_addr = inet_addr(_tcp_remote_ipaddr);
+	}
+
 	if (_ip == InternetProtocol::UDP) {
 
 		if ((_fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {