odhcp6c: implement RKAP: Reconfiguration Key Authentication Protocol

odhcp6c: implement RKAP: Reconfiguration Key Authentication Protocol

Problem: odhcp6c doesn't support authentication protocol other than
the Configuration Token protocol (RFC 3118).

https://www.ietf.org/archive/id/draft-ietf-dhc-rfc8415bis-12.html#name-reconfiguration-key-authent

Solution: implement Reconfiguration Key Authentication Protocol (RKAP)
and add the option to disable all authentication protocols (will discard
all reconfigure messages received).

Signed-off-by: Nicolas BESNARD <[email protected]>
Signed-off-by: Paul Donald <[email protected]>
Link: https://github.com/openwrt/odhcp6c/pull/106
Signed-off-by: Álvaro Fernández Rojas <[email protected]>