sort: fix hang with sort --compress

sort: fix hang with sort --compress

* NEWS: Document this.
* src/sort.c (UNCOMPRESSED, UNREAPED, REAPED): New constants.
(struct tempnode): New member 'state', to hold these constants.
The pid member is now undefined if state == UNCOMPRESSED.
(struct sortfile): Replace member 'pid' with member 'temp'.
(uintptr): Remove.
(proctab_hasher, proctab_comparator, register_proc, delete_proc):
Proctab entries are now struct tempnode *, not pid_t, to handle
the case where multiple tempnode objects correspond to the same
pid.  This avoids a race condition that can cause a hang.
(register_proc): Arg is now struct tempnode *, not pid_t.  All
callers changed.
(delete_proc): Set tempnode state to REAPED.
(create_temp_file): No need to set pid member here; it's now
done when the pid is known.
(maybe_create_temp, create_temp): Remove PPID arg.  Return struct
tempnode *, not char *.  All callers changed.
(maybe_create_temp): Set node state to UNCOMPRESSED or UNREAPED.
No need to set node->pid to 0.
(open_temp): Replace NAME and PID args with a single TEMP arg.
All callers changed.  Wait only for unreaped children.
(zaptemp): Wait for decompressor to finish before removing its
temporary-file input.  This avoids .nfsXXXX hassles with NFS
and fixes a race (leading to a hang) regardless of NFS.
(open_input_files): Adjust to new way of dealing with temp files
and their subprocesses.
* tests/Makefile.am (TESTS): Add misc/sort-compress-hang.
* tests/misc/sort-compress-hang: New file.