local: use common function to invoke parameter change handlers

This was missing in commit b69b648d.
sched: make sure scheduler parameter change handler is first

This is needed to allow other handlers to add new timers.
ntp: add function to restart transmit timer
ntp: restart timer when poll interval changes on reset
sched: improve time jump detection

To detect forward time jumps, use a timestamp made before calling
select() instead of the first timeout in the queue. Also, if the timeout
value is modified by select() (e.g. on Linux) use it to get a more
accurate estimate of the elapsed time.
doc: clarify description of -s option
ntp: return with status from functions sending packets
ntp: don't adjust polling interval when sending fails
ntp: don't stop online burst when sending fails

Don't stop online burst for unreachable sources until sending succeeds.
This is mainly useful with iburst when chronyd is started before the
network is configured.