change the nsec argument to timeout_add_nsec from int to uint64_t

change the nsec argument to timeout_add_nsec from int to uint64_t

you can only fit a couple of nanonseconds into an int, which limited
the usefulness of the api. worse, if a large nsec value was passed
in it could be cast to a negative int value which tripped over a
KASSERT at the top of timeout_add that ends up being called. avoid
this footgun by working in the bigger type and doing the same range
checks/fixes for other timeout_add wrappers.

ok claudio@ mvs@