sort: use more-consistent style with const

sort: use more-consistent style with const

* src/sort.c (proctab_hasher, proctab_comparator, stream_open, xfopen):
(open_temp, zaptemp, struct_month_cmp, begfield, limfield):
(find_unit_order, human_numcompare, numcompare, general_numcompare):
(count_tabs, keycompare, compare, compare_nodes, lock_node):
(unlock_node, queue_destroy, queue_init, queue_insert, queue_pop):
(write_unique, mergelines_node, check_insert, update_parent):
(merge_loop, sortlines, struct thread_args, set_ordering):
Prefer the style "T const" to "const T".
* gl/lib/heap.h (struct heap, heap_alloc): Likewise.
* gl/lib/heap.c (heap_default_compare, heapify_down, heapify_up):
(heap_alloc): Likewise.