sysstat version 12.6.0 final packaging.
Changelog added.
spec and lsm files updated.

This version adds a new metric (softnet backlog) to the software-based
network processing activity.
It also fixes several bugs concerning the recent interrupts statistics
rewritten code, and to metrics names used by sadf in the PCP output
format.

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