LXC: add support for the memory controller

Add support for the memory resource controller, useful to actually place
memory limits on instances.

Support is still optional, in case the kernel doesn't have it compiled
in, or -in the case of Debian- has it dependent on a kernel command-line
switch (cgroup_enable=memory specifically).

Signed-off-by: Faidon Liambotis <paravoid@debian.org>
Reviewed-by: Guido Trotter <ultrotter@google.com>