apparmor: Initial support for compressed policies

This patch allows policies to be compressed in userspace and be sent to
the kernel through the existing ".load" and ".replace" kernel interfaces.

The benefits of this approach are:
 - Save kernel time when loading policies
 - Allow userspace to provide a higher level of compression than the one
   provided by the kernel (ZSTD_CLEVEL_DEFAULT), thus saving space.
 - Allow small embedded systems to only store the compressed version of
   policies in userspace, saving memory.

Userspace-compressed policies improve system time by up to ~30% for big
profiles.

Signed-off-by: Maxime Bélair <maxime.belair@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

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