Userspace alarm service

Built off #A, this provides a basic alarm handler service similar to the
kernel's.  The main differences are that there are no semaphores and
only one tchain (interrupt source).

Instead of having to set up a full #A alarm for every little handler,
you can use the user alarm service, which will multiplex them in
userspace off a single kernel alarm.
SYS_abort_syscall (XCC)

Userspace can request the cancellation of a syscall.  If that sysc is
blocked on a rendez, such as when listening on a network chan, it will
return via error().  If not, nothing will happen.

Reinstall your kernel headers.
Fixes bug with cross-core kernel alarms

Bad pointer arithmetic.
Fixes tchain corruption

If you have multiple unsets of the same waiter, you would corrupt the
list.  I actually had this happen, though I'm not sure how.  The ksched
alarm was getting accidentally ripped off this list, causing all
processes to hang.  My hunch is that ping was setting and cancelling
alarms frequently, and perhaps userspace submitted concurrent requests
to unset its main alarm.
Alarm to abort syscalls, used in ping

Will need to reorganize a lot of userspace's libraries.  Stuff like
nsec2tsc() doesn't need to be in a benchmarking library.  Same goes for
the alarm stuff; benchutil is more of a "everything I can't put in
parlib that we want by default."

Also, I cleaned up ping a little, notably the interrupted read code and
the spacing between sends (and now we get reasonable RTTs).