sort: parallelize internal sort

This patch is by Gene Auyeung, Chris Dickens, Chen Guo, and Mike
Nichols, based off of a patch by Paul Eggert, Glen Lenker, et. al.,
with a basic heap implementation based off of the GDSL heap,
originally by Nicolas Darnis.

The number of sorts done in parallel is limited to the number
of available processors by default, or can be further restricted
with the --parallel option.

On a dual-die, 8 core Intel Xeon, results show sorting with
8 threads is almost 4 times faster than using a single thread.
Timings when sorting a 96MB file:
THREADS     TIME (s)
1            5.10
2            2.87
4            1.75
8            1.31

Single threaded sorting has also been improved,
especially for cheaper comparison operations:
COMMAND             BEFORE (s)  AFTER (s)
sort                 8.822       8.716
sort -g             10.336      10.222
sort -n              3.077       2.961
LANG=C sort          2.169       2.066

* bootstrap.conf: Add heap, pthread.
* coreutils.texi (sort): Describe the new --parallel option.
* gl/lib/heap.c: New file. Very basic heap implementation.
* gl/lib/heap.h: New file.
* gl/modules/heap: New file.
* src/Makefile.am: Add LIB_PTHREAD.
* src/sort.c: Include heap.h, nproc.h, pthread.h.
(MAX_MERGE): New macro.
(SUBTHREAD_LINES_HEURISTIC, PARALLEL_OPTION): New constants.
(MERGE_END, MERGE_ROOT): New constants.
(struct merge_node): New struct.
(struct merge_node_queue): New struct.
(sortlines temp): Remove declaration.
(usage, long_options, main): New option, --parallel.
(specify_nthreads): New function.
(mergelines): New signature, to emphasize the fact that the HI area
must be part of the destination.  All callers changed.
(sequential_sort): New function, renamed from sortlines. Merge in
the functionality of sortlines_temp.
(compare_nodes): New function.
(lock_node, unlock_node): New functions.
(queue_destroy): New function.
(queue_init): New function.
(queue_insert): New function.
(queue_pop): New function.
(write_unique): New function.
(mergelines_node): New function.
(check_insert): New function.
(update_parent): New function.
(merge_loop): New function.
(sortlines): Rewrite to support and use parallelism, with a new
signature. All callers changed.
(struct thread_args): New struct.
(sortlines_thread): New function.
(sortlines_temp): Remove.
(sort): New argument NTHREADS. All uses changed. Output moved to
mergelines_node.
(main): disable threading if we are sorting at random.
* tests/Makefile.am (TESTS): Add misc/sort-benchmark-random.
* tests/misc/sort-benchmark-random: New file.

Signed-off-by: Pádraig Brady <P@draigBrady.com>

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