shared: include root when canonicalizing conf paths

The conf_files_list family accepts an alternate root path to prefix all
directories in the list but path_strv_canonicalize_uniq doesn't use it.
This results in the suspicious behavior of resolving directory symlinks
based on the contents of / instead of the alternate root.

This adds a prefix argument to path_strv_canonicalize which will now
prepend the prefix, if given, to every path in the list. To avoid
answering what a relative path means when called with a root prefix
path_strv_canonicalize is now path_strv_canonicalize_absolute and only
considers absolute paths. Fortunately all users of already call
path_strv_canonicalize with a list of absolute paths.