Folk periodically talk about evolving the "struct completion"
support ... here's one:

   int wait_event (struct completion *x, signed long timeout_jiffies);

Returns zero if the event happened, nonzero if it timed out.
An example timeout action might be to cancel a request then
use wait_for_completion(x) to synchronize with that.

It's been behaving for me so far.  Questions from me:

- Is this appropriate to merge as-is?  The tradeoff I'm
   thinking of is code duplication; this patch avoids it
   ("smaller==better" rule-of-thumb) but maybe someone has
   real tuning knowledge, or a "standard" lk policy applies.
   Or there might be bugs or api transgressions.

- One routine was unavailable to modules; now exported.
   That seemed to just be an oversight.

This doesn't add an "interruptible wait" api mode, which
I'd expect some others might find a use for.  This patch
will let us clean up some dubious code in usbcore which
is more or less trying to do a wait_event().

Thanks in advance for any comments.

- Dave