tree-wide: stop using canonicalize_file_name(), use chase_symlinks() instead

Let's use chase_symlinks() everywhere, and stop using GNU
canonicalize_file_name() everywhere. For most cases this should not change
behaviour, however increase exposure of our function to get better tested. Most
importantly in a few cases (most notably nspawn) it can take the correct root
directory into account when chasing symlinks.

src/shared/condition.c

@@ -399,7 +399,7 @@ static int condition_test_path_is_mount_point(Condition *c) {
         assert(c->parameter);
         assert(c->type == CONDITION_PATH_IS_MOUNT_POINT);
 
-        return path_is_mount_point(c->parameter, AT_SYMLINK_FOLLOW) > 0;
+        return path_is_mount_point(c->parameter, NULL, AT_SYMLINK_FOLLOW) > 0;
 }
 
 static int condition_test_path_is_read_write(Condition *c) {