it's partition is configured "Shared=EXCLUSIVE" (which is redundant).

    after a job completed.

    size get queued).

    specific reservation.

salloc: notify terminal foreground process

This fixes another bug observed in salloc child process cleanup. I found
that some shells, e.g. zsh, do not forward all signals to their children.

The patch fixes the problem that
 * command_pid is still active but does not equal tpgid,
 * tpgid is not the same as salloc's process group,
 * tpgid is very unlikely to come from another process, since we block
   the suspend/TSTP signal,
 * signalling command_pid does not automatically imply that the active
   terminal foreground process is also signalled,
 * hence send a HUP to signify "death of controlling process".

This setup fixed the problem on zsh. I then went and tested a more complex setup:

Before:
-------
palu2:0 ~>ps  f -o pid,pgid,tpgid,ppid,stat,tty,cmd
  PID  PGID TPGID  PPID STAT TT       CMD
21117 21117 21597 21116 Ss   pts/9    -bash
21260 21260 21597 21117 Sl   pts/9     \_ ./slurm_build/git/src/salloc/salloc -v --time=00:01:00 -N17 zsh
21266 21266 21597 21260 S    pts/9         \_ zsh
21323 21323 21597 21266 S    pts/9             \_ /bin/bash
21397 21397 21597 21323 S    pts/9                 \_ -bin/tcsh
21526 21526 21597 21397 S    pts/9                     \_ /bin/sh
21597 21597 21597 21526 S+   pts/9                         \_ aprun -N1 -n17 sleep 12345
21601 21597 21597 21597 S+   pts/9                             \_ aprun -N1 -n17 sleep 12345

After the timeout:
------------------
palu2:0 ~>ps  f -o pid,pgid,tpgid,ppid,stat,tty,cmd
  PID  PGID TPGID  PPID STAT TT       CMD
21323 21323 21117     1 S    pts/9    /bin/bash
21397 21397 21117 21323 S    pts/9     \_ -bin/tcsh
21526 21526 21117 21397 S    pts/9         \_ /bin/sh

==> The 'dangerous' aprun terminal foreground process group 21597 has been removed, while the child
    subprocess groups 21323, 21397, and 21526 now exist as orph01_salloc-Bug-Fix-nested-terminal-foreground-process.diff
aned groups, to be cleaned up by init.

I noticed that the S_{JOB,STEP}_ALLOC_* comments in spank.h don't
document the return-by-value type as is done for the other spank_item
enumerations.