kernel/svc: Implement svcCreateEvent()

svcCreateEvent operates by creating both a readable and writable event
and then attempts to add both to the current process' handle table.

If adding either of the events to the handle table fails, then the
relevant error from the handle table is returned.

If adding the readable event after the writable event to the table
fails, then the writable event is removed from the handle table and the
relevant error from the handle table is returned.

Note that since we do not currently test resource limits, we don't check
the resource limit table yet.