Add AIX support

[Helflym]

This PR won't work as is until gofrs/flock#40 is merged.
However, it's possible in go.mod to specify a local directory in GOPATH to replace a Go module.
Therefore, once this PR has been merged, you can make it work on AIX by:

• cloning locally "github.com/gofrs/flock"
• patch it with the previous PR Add AIX support with fcntl gofrs/flock#40.
• add at the end of go.mod replace github.com/gofrs/flock => $path_to_your_local_gofrs

[urso]

Instead of adding more OSes to the _other files, I would prefer to change the build tag to +build !windows. So far it's the only OS that requires "more" special treatment. WDYT?

[Helflym]

Sure, that's a better idea as new patches won't be needed everytime a new OS appears. I'll make the changes