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.

doc/ntp.md

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+# Time
+
+The switch has no battery backed clock. It counts seconds from power on, and
+it can take the date and time from an NTP server once the network is up. The
+time is lost at every restart and set again at the first synchronisation.
+
+```
+ntp server <name|ip>     # default pool.ntp.org
+ntp interval <1-1440>    # minutes between two synchronisations, default 60
+ntp timezone <+hh:mm>    # offset from UTC, -12:00 to +14:00
+ntp dst off|eu|us        # daylight saving rule
+ntp on
+ntp off
+ntp                      # settings and time
+time                     # local date and time
+```
+
+The client speaks SNTP version 4 and synchronises at once when turned on,
+then after every interval. A server given by name is resolved through the DNS
+resolver before every synchronisation. An answer from an unsynchronised
+server is ignored, and a failed attempt is repeated after 30 seconds.
+
+The local time is UTC plus the time zone offset, plus one hour while daylight
+saving time applies:
+
+* `eu`: from 01:00 UTC on the last Sunday of March to 01:00 UTC on the last
+  Sunday of October;
+* `us`: from 02:00 local time on the second Sunday of March to 02:00 local
+  time on the first Sunday of November.
+
+The same settings, with the current local time and the last synchronisation,
+are in the Time card of the System page. Settings made there or on the
+console are kept only once the configuration is saved.