tree-wide: set SA_RESTART for signal handlers we install

We already set it in most cases, but make sure to set it in all others too, and
document that that's a good idea.

src/activate/activate.c

@@ -339,7 +339,7 @@ static void sigchld_hdl(int sig) {
 
 static int install_chld_handler(void) {
         static const struct sigaction act = {
-                .sa_flags = SA_NOCLDSTOP,
+                .sa_flags = SA_NOCLDSTOP|SA_RESTART,
                 .sa_handler = sigchld_hdl,
         };
 

src/nspawn/nspawn.c

@@ -3632,7 +3632,7 @@ static int run(int master,
 
         static const struct sigaction sa = {
                 .sa_handler = nop_signal_handler,
-                .sa_flags = SA_NOCLDSTOP,
+                .sa_flags = SA_NOCLDSTOP|SA_RESTART,
         };
 
         _cleanup_release_lock_file_ LockFile uid_shift_lock = LOCK_FILE_INIT;

src/udev/udevadm-monitor.c

@@ -143,7 +143,7 @@ static int adm_monitor(struct udev *udev, int argc, char *argv[]) {
 
         /* set signal handlers */
         act.sa_handler = sig_handler;
-        act.sa_flags = SA_RESTART;
+        act.sa_flags = SA_RESTART|SA_RESTART;
         sigaction(SIGINT, &act, NULL);
         sigaction(SIGTERM, &act, NULL);
         sigemptyset(&mask);