Parse prop171 options; refactor listener/port option code
Proposal 171 gives us a new syntax for parsing client port options. You can now have as many FooPort options as you want (for Foo in Socks, Trans, DNS, NATD), and they can have address:port arguments, and you can specify the level of isolation on those ports. Additionally, this patch refactors the client port parsing logic to use a new type, port_cfg_t. Previously, ports to be bound were half-parsed in config.c, and later re-parsed in connection.c when we're about to bind them. Now, parsing a port means converting it into a port_cfg_t, and binding it uses only a port_cfg_t, without needing to parse the user-provided strings at all. We should do a related refactoring on other port types. For control ports, that'll be easy enough. For ORPort and DirPort, we'll want to do this when we solve proposal 118 (letting servers bind to and advertise multiple ports). This implements tickets 3514 and 3515.