sd-dhcp6-client: Prevent setting and restarting of DHCPv6 client

Prevent modifications to index, MAC address, DUID and Information
Request while the DHCPv6 client is running.

Require the DHCPv6 client to be stopped first instead of always
unconditionally restarting it if the caller calls
sd_dhcp6_client_start() more than once. With this change, handling
of for example incoming Router Advertisments becomes much easier.