Change connectivity check to fall back to nc if curl doesn't like the --proxy-insecure argument

[phealy]

Curl on Ubuntu 16.04 doesn't support the --proxy-insecure argument, which causes checks to fail. This change puts a check in there - we call curl with the arguments we're using and --version. If curl supports all the arguments, it prints the version info and exits with rc 0. If it doesn't, it exits with rc 2 and the or check will use nc instead.