timer: Added usleep[_range] timer

After writing both documentation and a checkpatch rule explaining why the usleep API should never be used, it occurred to me that perhaps such an API should never be added :) - at least not in its previous form.

This iteration is similar, with the notable difference that now usleep has a "built-in slack" of 200%. This is analogous to msleep, which has a built-in slack of 0.4% (since it relies on legacy timers, which have a built-in slack of 0.4%). 200% slack is significantly greater than 0.4%, but the scale of usleep is also significantly different than that of msleep, and I believe 200% to be a sane default.

It is my opinion that this interface will most often mirror what developers actually intend - indeed some people who have begun trying to use the API raised this point -, however, I would like some input as it is possibly confusing that the API will "double your sleep" by default.

The usleep_range API is still included, since it provides an interface to override the "default slack" of 200% by providing an explicit range, or to allow callers to specify an even larger slack if possible.

This patch series is NOT based off of tip (read: it will conflict with the previous usleep patch in -mm) since it is slighly different, and Andrew Morton lamented to loss of the detailed changelog info in the previous iteration - it is included here.

This series also includes the "full set", that is
1: Adds the usleep[_range] API
2: Adds timers-howto documentation
3: Checkpatch: prefer usleep over udelay
4: Checkpatch: warn about unexpectedly long msleep's

*** Changes in v2
* Add "default slack" to usleep
* Fix missing usec->nsec for delta in do_usleep_range
* Fix Documentation typos
* Better checkpatch regex's