Processes to record topic generations before execution

The sigchld generation expresses the idea that, if we receive a sigchld
signal, the generation will be different than when we last recorded it. A
process cannot exit before it has launched, so check the generation count
before process launch. This is an optimization that reduces failing
waitpid calls.

src/exec.cpp

@@ -899,6 +899,7 @@ static bool exec_process_in_job(parser_t &parser, process_t *p, std::shared_ptr<
     }
 
     // Execute the process.
+    p->check_generations_before_launch();
     switch (p->type) {
         case INTERNAL_FUNCTION:
         case INTERNAL_BLOCK_NODE: {