MacUpdater: Retain file permissions

[spycrab]

Fixes updated bundles not being executable.

[delroth]

This does not handle the case where a new binary is added to the package, so at some point update compatibility will be broken.

Given that the update version incompatibility will need to be fixed at some point anyway, the choice is basically now vs. later (e.g. when a new binary is added). If it's postponed to later then there's the risk that people will not remember and break updates (possibly in a way that bricks installs), or that people who understand the auto-update system well are not involved with the project anymore. So IMO that's the "dangerous" kind of technical debt, and I would rather see it addressed now with a proper updater protocol version bump.

[spycrab]

While I think that addressing this issue properly as described in @delroth's post is both required and inevitable it's also important that we ship a fix to avoid confusing users and trapping them into using a broken feature. Especially since the proper fix might need some time before it's ready for shipping.

[riking]

Inserting a TODOs noting that (2) that this is a hack, (3) the likely failure condition of the hack, and (1) the exit condition to remove the hack, seems prudent.

// TODO: A new updater protocol version is required to properly mark executable files.  For
// now, copy executable bits from existing files. This will break for newly added executables.