Improve print service task and description

Using «print server» for the task name and description seems rather confusing, depending on the user expectations and experience. This tends to signal that this machine will be serving other systems, instead of making it obvious that it is also required for local printing.

Change the name to print service which is more generic, and does not have the same connotations as print server. Improve its description to match. Add a Provides for backwards compatibility. Move the task from server to user Section.

Closes: #696658 Ref: https://lwn.net/Articles/792952/