time: add timemono_since

time_mono()'s documentation references time_since(), which is nowhere
to be found.

A possible fix would be to just remove that stale reference.

Instead, this patch adds timemono_since(), which hopefully
captures the meaning of the original time_since().
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>