Revert "fixup! Using SIGSTOP/SIGCONT instead of mmap & sem_t to synchronize jobs"

This reverts commit f653fbfaf4.
It was meant for the major branch.

src/exec.cpp

@@ -513,6 +513,10 @@ void exec_job(parser_t &parser, job_t *j) {
         const bool pipes_to_next_command = !p->is_last_in_job;
         bool command_blocked = false;
 
+        //these semaphores will be used to make sure the first process lives long enough for the
+        //next process in the chain to open its handles, process group, etc.
+        //this child will block on this one, the next child will have to call sem_post against it.
+
         // The pipes the current process write to and read from. Unfortunately these can't be just
         // allocated on the stack, since j->io wants shared_ptr.
         //