dhcpv6-ia: use dhcp leasetime to set preferred/valid statefull lifetimes

dhcpv6-ia: use dhcp leasetime to set preferred/valid statefull lifetimes

Allow to set the preferred/valid lifetimes of IA_NA/IA_PD options based
on the configured dhcp leasetime.
DHCP leqasetime will be used to set the preferred/valid lifetimes in the
IA_NA/IA_PD options unless the preferred/valid lifetimes of the IPv6
address are smaller then the DHCP leasetime.
This will avoid IA_NA/IA_PD options being sent with infinite lifetimes
due to the IPv6 address having infinite preferred/valid lifetimes
like IPv6 ULA addresses.
While at it rename dhcpv4_leasetime into dhcp_leasetime as the leasetime
is used both for DHCPv4 and DHCPv6

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>