kern: Move some purely arithmetic routines to subr_time_arith.c.

Preparation for testing and fixing:

PR kern/58922: itimer(9): arithmetic overflow
PR kern/58925: itimer(9) responds erratically to c

kern: Move some purely arithmetic routines to subr_time_arith.c.

Preparation for testing and fixing:

PR kern/58922: itimer(9): arithmetic overflow
PR kern/58925: itimer(9) responds erratically to clock wound back
PR kern/58926: itimer(9) integer overflow in overrun counting
PR kern/58927: itimer(9): overrun accounting is broken

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