net: dhcpv4: client: Handle Pad option

Pad option (option code 0) can be present in between other options for
alignment. The option has a fixed 1-byte length (i. e. no length field),
therefore it did not fall under the common processing code for
unrecognized options (which include the length field at the second
byte). Therefore, not processing this option explicitly could disturb
other options processing, as the parser would wrongly interpret the next
option code as the length field. This commit adds Pad option handling to
fix the issue.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>