chase: replace path_prefix_root_cwd() with chaseat_prefix_root()

chase: replace path_prefix_root_cwd() with chaseat_prefix_root()

The function path_prefix_root_cwd() was introduced for prefixing the
result from chaseat() with root, but
- it is named slightly generic,
- the logic is different from what chase() does.

This makes the name more explanative and specific for the result of the
chaseat(), and make the logic consistent with chase().

Fixes https://github.com/systemd/systemd/pull/27199#issuecomment-1511387731.

Follow-up for #27199.