smack: label /etc/passwd and friends as '_' smack label when --with-smack-run-label...

smack: label /etc/passwd and friends as '_' smack label when --with-smack-run-label' is enabled

systemd-sysusers.service unit creates system users and groups and it
could update /etc/passwd, /etc/group, /etc/shadow and /etc/gshadow.
Those files should have '_' smack label because of accessibility.

However, if systemd has its own smack label using '--with-smack-run-label'
configuration, systemd-sysusers process spawned by systemd(pid:1) has
its parent smack label and eventually updated files also is set as its
parent smack label.

This patch fixes that bug by labeling updated files as '_' smack label
when --with-smack-run-label' is enabled.