shared: rename path_strv_canonicalize_absolute functions

Since 12ed81d9 path_strv_canonicalize_absolute leaves the search list
relative to the given root directory instead of resolving paths to their
true location as the name implies. To better reflect this behavior
rename to the less strongly worded path_strv_resolve.
test: unit test for using alternate roots with path_strv_resolve
conf-files: include root in returned file paths

This restores the original root handling logic that was present prior to
112cfb18 when path expansion moved to path_strv_canonicalize_absolute.
That behavior partially went away in 12ed81d9.

Alternatively all users of conf_files_list* could be updated to
concatenate the paths themselves as unit_file_query_preset did but since
no user needs the un-concatenated form that is pointless duplication.
test: ensure conf_files_list returns absolute paths
shared: fix search_and_fopen with alternate roots

Update for the current behavior of path_strv_resolve which now returns
paths relative to the given root, not the full absolute paths.