podman run --cidfile: fail if path exists

[edsantiago]

podman happily clobbers an existing cidfile:

# touch /tmp/mycid
# podman run --cidfile /tmp/mycid fedora date
Thu Mar 22 15:05:26 UTC 2018

docker fails with a helpful diagnostic:

/usr/bin/docker-current: Container ID file found, make sure the other container isn't running or delete /tmp/mycid.

This is a judgment call: one can imagine a UNIX old-timer ranting about overprotective tools and let me shoot myself in the foot if I want to and get off my lawn. I lean that way myself at times. Still... the cost of debugging a problem is much higher if the cidfile is silently overwritten. I would recommend following the docker convention. (Bonus: I think the diagnostic would be more readable as 'Will not overwrite existing cidfile "/tmp/mycid"')

[mheon]

This is probably 3 lines of code, so I vote we fix it.

[baude]

indeed, ill take it

[baude]

well if this was name that tune, I would lose. it took me six lines (3 lines in two places)

[rhatdan]

Yes lets fix it.