Enable support for Linux audit subsystem using sys-process/audit

Packages describing “audit” as local USE flag

Package“audit” Flag Description
sys-apps/policycoreutilsEnable support for sys-process/audit and use the audit_* functions (like audit_getuid instead of getuid())
sys-apps/systemdEnable support for sys-process/audit
sys-apps/util-linuxUse sys-process/audit to emit audit messages about system changes

All packages providing a “audit” USE flag (21)