dhcp: if first ntp invocation fails, set time manually and retry

[rpardini]

• some machines have no RTC at all, or an RTC with a failed battery
• in certain situations, ntpd fails with "Alarm Clock" when trying to do too-big a jump
• in this case, set system time to the date of the dhcpd binary and retry
• yet-another-case where systemd would have saved us from pain Signed-off-by: Ricardo Pardini [email protected]