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.

src/shared/install.c

@@ -1776,7 +1776,7 @@ UnitFileState unit_file_get_state(
 
 int unit_file_query_preset(UnitFileScope scope, const char *root_dir, const char *name) {
         _cleanup_strv_free_ char **files = NULL;
-        char **i;
+        char **p;
         int r;
 
         assert(scope >= 0);
@@ -1804,17 +1804,10 @@ int unit_file_query_preset(UnitFileScope scope, const char *root_dir, const char
         if (r < 0)
                 return r;
 
-        STRV_FOREACH(i, files) {
-                _cleanup_free_ char *buf = NULL;
+        STRV_FOREACH(p, files) {
                 _cleanup_fclose_ FILE *f;
-                const char *p;
 
-                if (root_dir)
-                        p = buf = strjoin(root_dir, "/", *i, NULL);
-                else
-                        p = *i;
-
-                f = fopen(p, "re");
+                f = fopen(*p, "re");
                 if (!f) {
                         if (errno == ENOENT)
                                 continue;