pidstat: Add new option to display timestamps in seconds since the Epoch

pidstat: Add new option to display timestamps in seconds since the Epoch

Previously, option -h used to display all activities horizontally on a
single line also displayed timestamps in second since the Epoch. Though
this was intended to be used by scripts, it was not necessarily a
desired feature (see #155).
This patch changes option -h so that time format remains unmodified (all
activities are displayed on a single line with no average statistics)
and adds a new option (-H) that tells pidstat to display time in seconds
since the Epoch. This option may be used separately from option -h.

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