linux: startup crash if ipv6 is disabled

[qdbp]

jupyter notebook with no arguments crashes if the following sysctl option is set

sudo sysctl net.ipv6.conf.all.disable_ipv6=1

with the error

OSError: [Errno 99] Cannot assign requested address

with no mention of ipv6. It's not intuitive that this is the cause, nor do I think this is the expected behaviour. Note that it works if you manually specify the ipv4 loopback address with --ip 127.0.0.1

If the sysctl setting is reset to 0, then jupyter starts again with no problems, giving me some confidence that ipv6 is the root cause.