CGroup Files: Add write_string control file method

This is a resend of a patchset that I sent last month, reworked to
remove some controversial locking proposals. All locking is now explicit.

This patchset provides:

1) A new write_string() cgroup file method that copies the user's data
to kernel space and invokes the relevant handler with the
nul-terminated kernelspace buffer

2) A new helper function, cgroup_lock_live_group(), which combines
taking the cgroup lock and checking the liveness of a cgroup, to allow
simplification of a common lock/check idiom in cgroup file handlers.

3) Conversion of several raw write handlers in cgroup, cpuset,
devcgroup and res_counter to use typed handlers and the new locking
specifications.

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

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