Hide current elpa directory behind a symlink

Simplifies rollbacks: revert symlink to previous directory.

Also,

* make directory names more intuitive: use $(date) for the _new_
directory, instead of the old one;
* write update patches to the new directory, instead of the old one;
* tee update summary to the new directory, so that it survives the
transient shell the update was run from.

Also, ask the augurs why 'test' has both -h and -L for symbolic links,

https://pubs.opengroup.org/onlinepubs/9799919799/utilities/test.html

and get no answer 🤷

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