lib: use O_CLOEXEC flag when opening EtherCAT master device

This flag specifies that the file descriptor should be closed when an
exec function is invoked. When a file descriptor is allocated (as with
open or dup), O_CLOEXEC bit is initially cleared on the new file
descriptor, meaning that descriptor will survive into the new program
after exec. Setting O_CLOEXEC avoid this survival of the desciptor in
the new program. And setting it at open() time is the only race-free
way to avoid accidentally leaking the fd via other threads that
concurrently do fork()+exec() (or similar, e.g. posix_spawn).

lib/common.c

@@ -97,9 +97,9 @@ ec_master_t *ecrt_open_master(unsigned int master_index)
             master_index);
 
 #ifdef USE_RTDM
-    master->fd = rt_dev_open(path, O_RDWR);
+    master->fd = rt_dev_open(path, O_RDWR | O_CLOEXEC);
 #else
-    master->fd = open(path, O_RDWR);
+    master->fd = open(path, O_RDWR | O_CLOEXEC);
 #endif
     if (EC_IOCTL_IS_ERROR(master->fd)) {
         EC_PRINT_ERR("Failed to open %s: %s\n", path,