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,
         };