src/oping.c: Exit early if no host could be resolved.

Otherwise "noping" would drop the user into an empty ncurses window,
which is very confusing, annoying and unexpected.
src/oping.c: Add median and 'n'th percentile, remove average and stddev.

Average and standard deviation are not very useful for network latency.
Median and (95th) percentiles are routinely used to measure network
performance, so provide them instead.
src/oping.c: Rewrite of the percentile code.

Rather than bucketizing RTTs and approximating the percentile, keep the
last 900 replies (15 minutes by default) in memory and calculate the
percentile from that.

This allows the "prettyping" graph to "move to the left", which is a much
nicer user experience IMHO.