AP_HAL_SITL: add O_CLOEXEC in places missing it

By opening with O_CLOEXEC we make sure we don't leak the file descriptor
when we are exec'ing or calling out subprograms. Right now we currently
don't do it so there's no harm, but it's good practice in Linux to have
it.

libraries/AP_HAL_SITL/Storage.cpp

@@ -13,7 +13,7 @@ using namespace HALSITL;
 void EEPROMStorage::_eeprom_open(void)
 {
     if (_eeprom_fd == -1) {
-        _eeprom_fd = open("eeprom.bin", O_RDWR|O_CREAT, 0777);
+        _eeprom_fd = open("eeprom.bin", O_RDWR|O_CREAT|O_CLOEXEC, 0777);
         assert(ftruncate(_eeprom_fd, HAL_STORAGE_SIZE) == 0);
     }
 }

libraries/AP_HAL_SITL/sitl_gps.cpp

@@ -902,7 +902,7 @@ void SITL_State::_update_gps_file(const struct gps_data *d)
 {
     static int fd = -1;
     if (fd == -1) {
-        fd = open("/tmp/gps.dat", O_RDONLY);
+        fd = open("/tmp/gps.dat", O_RDONLY|O_CLOEXEC);
     }
     if (fd == -1) {
         return;

libraries/SITL/SIM_JSBSim.cpp

@@ -173,7 +173,7 @@ bool JSBSim::start_JSBSim(void)
     }
 
     int p[2];
-    int devnull = open("/dev/null", O_RDWR);
+    int devnull = open("/dev/null", O_RDWR|O_CLOEXEC);
     if (pipe(p) != 0) {
         AP_HAL::panic("Unable to create pipe");
     }

libraries/SITL/SIM_last_letter.cpp

@@ -55,7 +55,7 @@ void last_letter::start_last_letter(void)
     if (child_pid == 0) {
       // in child
       close(0);
-      open("/dev/null", O_RDONLY);
+      open("/dev/null", O_RDONLY|O_CLOEXEC);
       for (uint8_t i=3; i<100; i++) {
           close(i);
       }