nettools does not expose the original lease lifetime. It's a missing
API. Instead, it only exposes the timestamp when the lease will expire.

As a workaround, we calulate the timestamp by subtracting the current
timestamp from the expiration timestamp, assuming that the lease was
received just now. However, it was not received *exactly* now, but a
few milliseconds before. Hence, the calculated timestamp is not exact
here and likely a few milliseconds less then the actual (full integer)
value.

Account for that by rounding the value to the second.

and assert values exported as dhcp options are utf8 compliant

Drop it from the functions for extracting the dhcp options from the
lease: it was just used for the logging, but now we log all the options
once, at the end of the process.

as these are already logged in common dhcp-client code. Moreover, now
the log level of the lease options will move from INFO to DEBUG.

as these are already logged in common dhcp-client code.

Each plugin logged the options: just do that on dhcp state change and do
in common code.
Log the options at INFO level for all the plugins. This partially reverts
the effects on the internal plugin of the commit:
97ce488f5f50 ('dhcp/internal: decrease logging level when
retrieving dhcp options')

also fix the lease time: it should be the time in seconds that the lease
lasts

  $ find * -type f |xargs perl contrib/scripts/spdx.pl
  $ git rm contrib/scripts/spdx.pl