Check that the pidfile is not already present before overriding it

[byroot]

It fix the issue reported here: rails/rails#5534

At first I tried to write some test, but because of the exit(1) it's not that easy. If someone have a idea of how to write some test about that...

[JeronimoColon]

Thanks for taking care of this. Hopefully it gets accepted cause it's a pain to loose the pid.

[raggi]

I would like to do several things with this before merge:

• Check if the PID is running, if it is, then abort
• If the PID is not running, then boot and replace the pidfile.
• Add tests.

[byroot]

Sure, I'll try to find a moment to do that this week end.

[byroot]

Here we are.

The code now check if the process is still running / dead / or owned by another user.
With associated tests.

I also tried to test the check_pid!method, but I don't know how to test exit and STDERR.puts calls.

[travisbot]

This pull request passes (merged 3b9410e into 6141192).

[raggi]

Merged, thank you!

[raggi]

FTR, I made a minor change:

edc8b92