Added support for cycling over multiple A records supplied by RR DNS.

This adds support for cycling over multiple A record addresses if supplied by DNS, if the command line flag is set.  Default behavior is the same as it was previously.
This isn't thread-safe. Multiple threads can read the same value from `cfg.addridx` and subsequently use the same record. This isn't so bad, but a thread can pause between the `cfg.addridx++` and the `if (cfg.addridx ...`. If another thread runs `cfg.addridx` at the same time, no threads will ever observe `cfg.addridx == cfg.numaddr`, and eventually you'll end up accessing some page you shouldn't be.