Prevent stopping non-persistent jails twice

[grembo]

This aims to address #152.

The problem at hand isn't really to prevent pot stop from
running in parallel (this could be handled on a global level),
but to prevent running "_js_stop" in pot start, after a
non-persistent jail was stopped.

The solution is very simple, as the use case this is about is:

1. pot stop is called
2. jail -r is called
3. pot start continues and calls "_js_stop"

By removing a file (and touching it when running "_js_stop")
this can be prevented without having to use real lock files.

Note: Given the number of files pot creates, it might make sense
to start using a subdirectory called /tmp/pot.

[grembo]

@pizzamig I hope I don't duplicate your work here - it's very little code though, so the damage done is limited :)

[pizzamig]

I didn't start yet, I was enjoying the hot day today and I was planning to prepare it tomorrow.
However, I'll run some tests on it.
Even if touching a file is not atomic (a small time window when the jail exits and pot stop is in execution) I think it's good enough, using lockf seems a major rework for a small benefit

[grembo]

It was so hot today, the only thing I could move was my fingers :)