Smack: Test if smack is enabled before mounting

Since on most systems with xattr systemd will compile with Smack
support enabled, we still attempt to mount various fs's with
Smack-only options.

Before mounting any of these Smack-related filesystems with
Smack specific mount options, check if Smack is functionally
active on the running kernel.

If Smack is really enabled in the kernel, all these Smack mounts
are now *fatal*, as they should be.

We no longer mount smackfs if systemd was compiled without
Smack support. This makes it easier to make smackfs mount
failures a critical error when Smack is enabled.

We no longer mount these filesystems with their Smack specific
options inside containers. There these filesystems will be
mounted with there non-mount smack options for now.