Allow users to opt-out of sending the UDP ping and improve error handling when the sendto syscall fails

Allow users to opt-out of sending the UDP ping and improve error handling when the sendto syscall fails.

This is a first attempt to improve the metrics opt-out and rescue the exceptions thrown when the sendto syscall fails, for example when it is blocked by a firewall or if there is no Internet connectivity.

I changed the boolean preference to three symbols: none, ping and full

See also: https://github.com/capistrano/stats/pull/1

Signed-off-by: Clemens Gruber <clemensgru@gmail.com>