(inittables, sort_buffer_size, getmonth, mergefps,

(inittables, sort_buffer_size, getmonth, mergefps,
first_same_file, merge, sort, main): Use size_t for indexes into arrays.
This fixes some unlikely havoc-wreaking bugs (e.g., more than INT_MAX
temporary files).
(getmonth, keycompare, compare): Rewrite to avoid need for alloca,
thus avoiding unchecked stack overflow in some cases.  As a side
effect this improve the performance of "sort -M" by a factor of 4
on my benchmarks.