odhcp6c: add sendopts config support and update to latest git HEAD

odhcp6c: add sendopts config support and update to latest git HEAD

Add sendopts config support allowing to add options in sent DHCPv6 packets.

Options can be configured as follows :
	uci set network.wan6.sendopts="sntpservers:3001:3001::1,3001:3001::2 11:00000000000000000000006674692F 0x3e8:ABCDEF"

Based on a patch by Frank Andrieu <fandrieu@gmail.com>

See https://git.openwrt.org/?p=project/odhcp6c.git;a=commit;h=510aaf6d528210c5e8a6159f9b80b32615e88c5f
for a more detailed description.

Latest git changes :
	1f93bd4 dhcpv6: rework option passthrough logic
	a477e95 odhcp6c: rework userclass and vendorclass command handling
	510aaf6 odhcp6c: add -x opt:val support
	ab75be1 treewide: update copyrights to 2018
	f3a4609 odhcp6c: let odhcp6c_add_state return a success/failure indication

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