Preserve permission bits on updateself

[alexkeizer]

Tries to preserve any changed permissions on the bitwarden.sh script when updating it (with updateself).
For example, I like to have my instances bitwarden.sh to be group executable, so this change would keep it group executable after updates.

One thing is that I'm not sure whether the --reference option to chmod is specific to the GNU implementation. Is that is a concern?

[sincerelysaucy]

The normal method of controlling permissions is to specifically set them, not to merely copy them and rely on the history of permissions of the previous deployment. Relying on history that is out of the control of the deployment tool creates uncertainty of deployment, a dangerous practice for a security tool.

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