Count short read slop when seeking

Count short read slop when seeking

* src/buffer.c (short_read_slop): New static var.
(get_archive_status): Treat anything other than fifos and sockets
as potentially seekable; they’ll tell us if they aren’t, whereas
fifos and sockets cannot be seekable.  Check named files for
initial offset too, to deal with names like /dev/stdin.
Do not worry about start_offset’s value if !seekable_archive,
as it won’t be used.  Use short_read_slop.
(short_read, try_new_volume, simple_flush_read, _gnu_flush_read):
Set short_read_slop.