odhcpd: add option for setting preferred lifetime

odhcpd: add option for setting preferred lifetime

"valid_lft" and "preferred_lft" are different. If the "preferred_lft"
is expired the prefix should be avoided in source prefix selection.
However, the interface is allowed to still receive downstream traffic.

preferred_lfetime:
  Limit for preferred lifetime of a prefix

If you want the old behavior, you have to set preferred_lifetime to
the same value as leasetime.

Signed-off-by: Nick Hainke <vincent@systemli.org>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>