core: mount and initialize Smack

SMACK is the Simple Mandatory Access Control Kernel, a minimal
approach to Access Control implemented as a kernel LSM.

The kernel exposes the smackfs filesystem API through which access
rules can be loaded. At boot time, we want to load the access rules
as early as possible to ensure all early boot steps are checked by Smack.

This patch mounts smackfs at the new location at /sys/fs/smackfs for
kernels 3.8 and above. The /smack mountpoint is not supported.
After mounting smackfs, rules are loaded from the usual location.

For more information about Smack see:
  http://www.kernel.org/doc/Documentation/security/Smack.txt

README

@@ -44,6 +44,8 @@ REQUIREMENTS:
           CONFIG_NET
           CONFIG_SYSFS
 
+        Linux kernel >= 3.8 for Smack support
+
         Udev will fail to work with the legacy layout:
           CONFIG_SYSFS_DEPRECATED=n