interface.c: split out free_prefix_list

The prefix list from the interface is going to be used to generate
a new list of prefixes to send in an advert.  As such, it makes
since to reuse a common free_prefix_list function to clean up both
the iface->prefix list and the prefix list generated in send.c.
send.c: add build_prefix_list function

This function is currently only coping the prefix list, but is soon
going to be taking over the auto-prefix "::/64" code in gram.y.
gram.y: remove prefix/autoselect code

This code doesn't belong in the config file parse.  It should be
much simpler to configure auto-selected prefixes outside of a parser.