units: replace symlinks in units/user/ by real files

We already *install* those as real files since de78fa9b.
Meson will start to copy symlinks as-is, so we would get dangling symlinks in
/usr/lib/systemd/user/.

I considered the layout in our sources to match the layout in the installation
filesystem (i.e. creating units/system/ and moving all files from units/ to
units/system/), but that seems overkill. By using normal files for both we get
some duplication, but those files change rarely, so it's not a big downside in
practice.

Fixes #9906.