reconfigure: implement DHCP reconfiguration

reconfigure: implement DHCP reconfiguration

Problem: To update DHCP parameters/options, the process needed to be
restarted with the new arguments. It makes odhcp6c not very flexible,
making it impossible to keep track of the total packet statistics.

Solution: Add a new ubus method to reconfigure DHCP without restarting
the process.

Signed-off-by: Nicolas BESNARD <nico.besnard31@gmail.com>
Signed-off-by: Paul Donald <newtwen+github@gmail.com>
Link: https://github.com/openwrt/odhcp6c/pull/106
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>