- Add migration code to handle upgrade from old version of the man
package not supporting alternative path move (boo#1175919).

Bug was reopened by someone who skipped the Tumbleweed snapshot that changed libexecdir.

At this point, I don't think that encumbering spec file just to ensure migration path from older Tumbleweed upgrades worth it - Tumbleweed being fast moving with informed users.

But it made me think that migration path from Leap to Tumbleweed is also broken; And the one from Leap 15.2 to the next Leap version (or whatever the name becomes) will be broken as well.

So it might actually be good to add migration stuff. Feel free to decline though or do it differently, I'm not a fan of such things.

I've tested only:

- Upgrade from freshly installed old package (the one with old libexecdir and old update-alternatives calls, rebuilt here: https://build.opensuse.org/package/show/home:1Antoine1:branches:Base:System/man).
- Upgrade from current man package freshly installed.