Don't ignore -S if input is a pipe. Bug report by Michael McFarland in

Don't ignore -S if input is a pipe.  Bug report by Michael McFarland in
<http://mail.gnu.org/archive/html/bug-coreutils/2003-09/msg00008.html>.

(sort_buffer_size): Omit SIZE_BOUND arg.  Compute the
size_bound ourselves. if an input file is a pipe and the user
specified a size, use that size instead of trying to guess the
pipe size.  This has the beneficial side effect of avoiding the
overhead of default_sort_size in that case.  All callers changed.
(sort): Remove static var size; now done by sort_buffer_size.