Add configuration option PreferIPv6?

First take on proposal 186 "multiple OR ports" (legacy/trac#3785 (moved)) adds a second IP address to the routerinfo_t struct making it possible for a bridge to bind to an IPv6 address too. Both addresses are announced in the descriptor, as per proposal 186.

An OP which finds two IP addresses in a bridge descriptor and has both of these mentioned in Bridge config lines will have to prefer one over the other. One way would be to add an option PreferIPv6 0|1 (with a default of 0). Another way would be to prefer the one mentioned first in the config file (if the information of the order of config lines are kept after parsing the config file).