selinux: Don't attempt to load policy in initramfs if it doesn't exist

Currently on at least Fedora, SELinux policy does not come in the
initramfs.  systemd will attempt to load *both* in the initramfs and
in the real root.

Now, the selinux_init_load_policy() API has a regular error return
value, as well as an "enforcing" boolean.  To determine enforcing
state, it looks for /etc/selinux/config as well as the presence of
"enforcing=" on the kernel command line.

Ordinarily, neither of those exist in the initramfs, so it will return
"unknown" for enforcing, and systemd will simply ignore the failure to
load policy.