makefile: remove faq.php rule
cmdmon: don't create socket when cmdport is 0
sched: exit with fatal message when there is nothing to do

With cmdport 0 and port 0, it's now possible that there is no descriptor
watched or timer running, i.e. chronyd doing nothing and only waiting to
be terminated. Replace the assertion with LOG_FATAL to exit properly.
Update configuration examples
doc: update FAQ
doc: update initstepslew description
doc: update README
doc: update NEWS