cgroup map files: Add a key/value map file type to cgroups

These patches add a new cgroup control file output type - a map from
strings to u64 values - and make use of it for the memory controller
"stat" file.

It is intended for use when the subsystem wants to return a collection
of values that are related in some way, for which a separate control
file for each value would make the reporting unwieldy.

The advantages of this are:

- more standardized output from control files that report
similarly-structured data

- less boilerplate required in cgroup subsystems

- simplifies transition to a future efficient cgroups binary API

Signed-off-by: Paul Menage <menage@google.com>

--