May I pause/stop Startify from creating symlinks?

**May I opt to stop creating/updating the `__LAST__` auto-save session?** Simply, to stop Startify from creating such symlink? Here is why:

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Out of the blue, the Dropbox client on one of my Windows 10 machine refused to upload local changes but continued to fetch remote updates seamlessly. One week later, the Dropbox support people gave me a simple solution ==> to avoid using symlink across all subfolders in `~/Dropbox/`. I used the following command-line arguments to _look for_ symlink "files", and found `__LAST__` to be the only one that exists on all my systems.
* On MacOS/Linux: `find ~/Dropbox -type l -exec ls -lah {} \;`;
* On Windows:
* Through Command Prompt: `dir "%HOMEPATH%\Dropbox" /A:L /S`
* Through WSL (windows subsystem for Linux): `find /mnt/c/Users/YourUserName/Dropbox -type l -exec ls -lah {} \;` (Need to update the username bit);

Across all my Vim installation on various machines, I have defined an environmental variable with an absolute path to a sub-folder in `~/Dropbox`.
```
let g:startify_session_dir=$Startify_home
```
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Quote from Dropbox support:
> In general, we recommend users move the actual folders into the Dropbox folder and then symlink back to the original locations. Please just keep in mind that we don't generally support the use of symlinks due to the issues that can arise when they are added to Dropbox.
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Thanks a lot!

Start by tracing the session handling that creates or updates the `__LAST__` symlink and review the existing `g:startify_session_dir` configuration mentioned in the issue. Check how Startify options are documented and tested, if applicable. Done means users can opt out of creating or updating `__LAST__` while normal session behavior remains available.

Written by the indexing model from the issue text.