Revert "selinux: split up mac_selinux_have() from mac_selinux_use()"

Revert "selinux: split up mac_selinux_have() from mac_selinux_use()"

This reverts commit 6355e75610a8d47fc3ba5ab8bd442172a2cfe574.

The previously mentioned commit inadvertently broke a lot of SELinux related
functionality for both unprivileged users and elogind instances running as
MANAGER_USER.  In particular, setting the correct SELinux context after a User=
directive is used would fail to work since we attempt to set the security
context after changing UID.  Additionally, it causes activated socket units to
be mislabeled for elogind --user processes since setsockcreatecon() would never
be called.

Reverting this fixes the issues with labeling outlined above, and reinstates
SELinux access checks on unprivileged user services.