Enable system call restarting

When the SIGCHLD signal is received, it aborts interruptible system
calls. To avoid having to retry the system calls on the client side on
such an error, enable automatic system call restarting.

Author: Jugst3r

src/os/unix/process-wrappers.c

@@ -129,7 +129,7 @@ int __gnatcoll_init_sigchld_monitoring()
    struct sigaction action;
    action.sa_handler = __gnatcoll_write_on_sigchld;
    sigemptyset(&action.sa_mask);
-   action.sa_flags = 0;
+   action.sa_flags = SA_RESTART;
    return sigaction (SIGCHLD, &action, NULL);
 }