migrate to new SELinux api

migrate to new SELinux api

Using hard-coded access vector ids is deprecated and can lead to issues with custom SELinux policies.
Switch to `selinux_check_access()`.

Also use the libselinux log callback and log if available to audit.
This makes it easier for users to catch SELinux denials.

Drop legacy shortcut logic for passwd, which avoided a SELinux check if uid 0 changes a password of a user which username equals the current SELinux user identifier.
Nowadays usernames rarely match SELinux user identifiers and the benefit of skipping a SELinux check is negligible.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>