fire timer only on server select timeout
improve clock precision hack

OpenSSL with ntpd calls poll() before ntpd measures the clock precision,
which breaks the hack. Change the code to disable the hack only when
poll()/select() was called with NTP socket or a timer, or more than 0.1
seconds have passed.
add support for fd timers
add close()
update broadcast check in Network::send()
rename server socket variable