allow for network checks to be skips in setup while testing using SKIP_NETWORK_CHECKS=1
have nsd bind to the network interaface that is connected to the Internet, rather than all non-loopback network interfaces

hopefully fixes #121; thanks for the help @sfPlayer1
put a 15-second timeout in asking icanhazip.com for our IP address, although this limit does not seem to actually work (i.e. if I set the limit to 5 seconds, curl still hangs 10+ when I turn off my network connection)
dont ask the user for the machine's IP address if we can be sure our guess is right (trust icanhazip to give us the right answer)