sd-event: expose the event loop iteration counter via sd_event_get_iteration()

sd-event: expose the event loop iteration counter via sd_event_get_iteration()

This extends the existing event loop iteration counter to 64bit, and exposes it
via a new function sd_event_get_iteration(). This is helpful for cases like
issue #3612. After all, since we maintain the counter anyway, we might as well
expose it.

(This also fixes an unrelated issue in the man page for sd_event_wait() where
micro and milliseconds got mixed up)