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).

src/shared/bus-unit-util.c

@@ -2607,6 +2607,7 @@ static int bus_append_unit_property(sd_bus_message *m, const char *field, const
                               "RefuseManualStop",
                               "AllowIsolate",
                               "IgnoreOnIsolate",
+                              "SurviveFinalKillSignal",
                               "DefaultDependencies"))
                 return bus_append_parse_boolean(m, field, eq);