acmego - check for existing instances

[skuzzymiglet]

acmego doesn't report exisiting acmego instances, which can cause weird behaviour (gofmt text doubling). Maybe there should be a lock file? Or just a pidof-like check?

[rsc]

It's OK to have more than one as long as they are attached to different acmes.
I've been meaning to figure out some better way to detect the conflict.
Back when goimports was slow sometimes you'd do a Put and keep editing
and then acmego would show up and change bytes, which was no good either.
If we fix that, it would fix double acmego too.