Function: timer-activate

Insert TIMER into `timer-list'.
If TRIGGERED-P is t, make TIMER inactive (put it on the list, but
mark it as already triggered). To remove it, use `cancel-timer'.

REUSE-CELL, if non-nil, is a cons cell to reuse when inserting
TIMER into `timer-list' (usually a cell removed from that list by
`cancel-timer-internal'; using this reduces consing for repeat
timers). If nil, allocate a new cell.