Fix bad job control msg on trapped SIGINT and set -b (re: fc655f1)

When running an external command while trapping Ctrl+C via SIGINT,
and set -b is on, then a spurious Done job control message is
printed. No background job was executed.

    $ trap 'ls' INT
    $ set -b
    $ <Ctrl+C>[file listing follows]

    [1] +  Done                    set -b

In jobs.c (487-493), job_reap() calls job_list() to list a running
or completed background job, passing the JOB_NFLAG bit to only
print jobs with the P_NOTIFY flag. But the 'ls' in the trap is not
a background job. So it is getting the P_NOTIFY flag by mistake.

In fact all processes get the P_NOTIFY flag by default when they
terminate. Somehow the shell normally does not follow a code path
that calls job_list() for foreground processes, but does when
running one from a trap. I have not yet figured out how that works.

What I do know is that there is no reason why non-background
processes should ever have the P_NOTIFY flag set on termination,
because those should never print any 'Done' messages. And we seem
to have a handy P_BG flag that is set for background processes; we
can check for this before setting P_NOTIFY. The only thing is that
flag is only compiled in if SHOPT_BGX is enabled, as it was added
to support that functionality.

For some reason I am unable to reproduce the bug in a pty session,
so there is no pty.sh regression test.

src/cmd/ksh93/sh/jobs.c:
- Rename misleadingly named P_FG flag to P_MOVED2FG; this flag is
  not set for all foreground processes but only for processes moved
  to the foreground by job_switch(), called by the fg command.
- Compile in the P_BG flag even when SHOPT_BGX is not enabled. We
  need to set this flag to check for a background job.
- job_reap(): Do not set the P_NOTIFY flag for all terminated
  processes, but only for those with P_BG set.

src/cmd/ksh93/sh/xec.c: sh_fork():
- Also pass special argument 1 for background job to job_post() if
  SHOPT_BGX is not enabled. This is what gets it to set P_BG.
- Simplify 5 lines of convoluted code into 1.

Resolves: https://github.com/ksh93/ksh/issues/481

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2022-07-14:
+
+- Fixed a bug that caused a spurious "Done" message on the interactive shell
+  when an external command was run as a foreground job from a SIGINT trap.
+
 2022-07-12:
 
 - The .sh.level variable can now only be changed within a DEBUG trap. When