Write fascist_firewall_choose_address_ls() and use it in hs_get_extend_info_from_lspecs()

Currently, the address choice logic is:

• if we have an IPv6 address and can reach the ls IPv6 address, and prefer IPv6, use it
• if we have an IPv4 address and can reach the ls IPv4 address, use it

But it needs to be:

• if we have both addresses and can reach both, then use whatever we prefer
• if we have one address and can reach it, use it

This doesn't matter until clients put IPv6 addresses in the link specifier.