ntp: keep the time from an NTP server

The switch has no battery backed clock, only a seconds counter that starts
at zero on every boot. An SNTP client keeps UTC as the time of the last
answer plus the seconds counted since: "ntp server <name|ip>", "ntp interval
<minutes>", "ntp timezone <+hh:mm>", "ntp dst off|eu|us", "ntp on|off",
"ntp" and "time". A server given by name goes through the DNS resolver, an
answer from an unsynchronised server is ignored, and a failed attempt is
repeated after 30 seconds.

The local time adds the time zone offset and, while the European or United
States rule says so, one hour. The date is split with a shift and subtract
division and a walk over the years, so no 32-bit division from the library
ends up in the common area.

The System page gets the NTP settings and shows the local time.

udp_apps.h

@@ -4,6 +4,7 @@
 #include "dhcp.h"
 #include "syslog.h"
 #include "dns.h"
+#include "ntp.h"
 
 void udp_callbacks(void);