Make dispatch() run through a list of protocol structures to select against, rather...

Make dispatch() run through a list of protocol structures to select against, rather than running through the interface list and then special casing other stuff.   This simplifies things substantially.   add_protocol() function allows new protocols to be selected against.   Timeouts now pass a void * instead of a struct interface_info *.   Move do_packet into options.c so that options.c isn't pulled into dhcrelay.