pid1: add SurviveFinalKillSignal= to skip units on final sigterm/sigkill spree

pid1: add SurviveFinalKillSignal= to skip units on final sigterm/sigkill spree

Add a new boolean for units, SurviveFinalKillSignal=yes/no. Units that
set it will not have their process receive the final sigterm/sigkill in
the shutdown phase.

This is implemented by checking if a process is part of a cgroup marked
with a user.survive_final_kill_signal xattr (or a trusted xattr if we
can't set a user one, which were added only in kernel v5.7 and are not
supported in CentOS 8).