Remove NESTED and WAIT_BY_PROCESS

Now jobs are aware of their parent jobs, and can interrogate those jobs,
to determine if every job in the chain is fully constructed.
Remove flags and the static stacks that manipulated them.

src/proc.cpp

@@ -1090,7 +1090,7 @@ void job_t::continue_job(bool send_sigcont) {
             // is also the only place that send_sigcont is false. parent_job.is_constructed()
             // must also be true, which coincides with WAIT_BY_PROCESS (which will have to do
             // since we don't store a reference to the parent job in the job_t structure).
-            bool block_on_fg = send_sigcont && !get_flag(job_flag_t::WAIT_BY_PROCESS);
+            bool block_on_fg = send_sigcont && job_chain_is_fully_constructed();
 
             // Wait for data to become available or the status of our own job to change
             while (!reader_exit_forced() && !is_stopped() && !is_completed()) {