condition: add ConditionSecurity

condition: add ConditionSecurity

Using ConditionSecurity a unit can depend on a security module being
enabled/disabled. For now the only recognized security module is SELinux.

I'd like to use this feature for a unit that creates /.autorelabel if
SELinux is disabled, to ensure a relabel is done automatically when the
system is later rebooted with SELinux enabled.