Set $0 to more easily find gazelle in ps(1)

[frioux]

trafficstars

In starman $0 is set to starman master and starman worker respectively. This is super handy when doing debugging. This patch adds the same feature to Gazelle.

[frioux]

Any reason not to merge this?

[kazeburo]

I think that it is better to rewrite process name with --process-name option. And By default, it is better not to rewrite for backward compatibility.