sar: Add discard I/O metrics to "sar -b" output

sar: Add discard I/O metrics to "sar -b" output

"sar -b" statistics are calculated as the sum of individual
statistics for each whole device in the system. Since discard I/Os are
now separated from writes in the kernel, take them into account when
calculating "sar -b" statistics.
New fields are added to "sar -b" output: dtps and bdscd/s.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>