updater needs to support use of symlinks

In legacy/trac#11429 (moved), the "second Tor Browser dock icon on Mac OS" problem was solved by using a different Info.plist for the browser app bundle that is used by meek. Symlinks are used to create the second app bundle (to avoid duplicating all of the browser binaries).

Unfortunately, based on code inspection, it looks like the Mozilla updater implementation does not support symlinks. I will confirm next week by doing some testing, but assuming I am correct we will need to either:

(1) Patch the updater code to handle symlinks.

or

(2) Avoid symlinks in the package (but I do not have a better idea for solving legacy/trac#11429 (moved)).

I have not yet scoped the work for (1); hopefully it will not be too much work. If we always remove symlinks before applying an update, it may be enough to add support for a "addsymlink" verb in the update manifest.