Include physmem.h.

Include physmem.h.
(SORTALLOC, mergealloc, LINEALLOC): Remove.
(sortalloc): Default to zero at program startup.
(SORTALLOC_MIN, SORTALLOC_DEFAULT_MIN): New macros.
(usage, main): Add support for new -S SIZE option.
(specify_sort_size, default_sort_size): New functions.
(initlines): Do not let alloc exceed limit.
(findlines): Likewise.
(checkfp, mergefps, sort): Use sortalloc to size everything
else, instead of relying on precomputed sizes.