CGroup API files: Various cleanup to CGroup control files

This patchset is a roll-up of the non-contraversial items of the
various patches that I've sent out recently, fixed according to the
feedback received.

In summary they are:

- general rename of read_uint/write_uint to read_u64/write_u64

- use these methods for cpusets and memory controller files

- add a read_map cgroup file method, and use it in the memory
  controller

- move the "releasable" control file to the debug subsystem

- make the debug subsystem config option default to "n"

The only user-visible changes are the movement of the "releasable"
file and the fact that some write_u64()-based control files are now
more forgiving of additional whitespace at the end of their input.

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

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