sort: integer overflow checks in thread counts, etc.

sort: integer overflow checks in thread counts, etc.

* src/sort.c (specify_nthreads, merge_tree_init, init_node):
(queue_init, sortlines, struct thread_args, sort, main):
Use size_t, not unsigned long int, for thread counts, since thread
counts are now used to compute sizes.
(specify_nthreads): Check for size_t overflow.
(merge_tree_init, sort): Shorten name of local variable, for
readability.
(merge_tree_init): Move constants next to each other in product,
so that the constant folding is easier to see.
(init_node): Now static.  Add 'restrict' only where it might
be helpful for compiler optimization.
(queue_init): 2nd arg is now nthreads, not "reserve", which is
a bit harder to follow.  All uses changed.
(struct thread_args): Rename lo_child to is_lo_child, so that
it's obvious to the reader when we're talking about this boolean
as opposed to the new lo_child member of the other structure.
All uses changed.
(sort): Remove unused local variable end_node.
(main): Don't allow large thread counts to cause undefined behavior
later, due to integer overflow.