mkdir: drop mkdir_errno_wrapper(), use mkdirat_errno_wrapper() instead

Let's reduce our code duplication, and let's focus on using xyzat()
style APIs more, hence drop mkdir_errno_wrapper() and stick to
mkdirar_errno_wrapper() wherever we can, it's a true superset of
functionality after all.

src/shared/mkdir-label.c

@@ -30,13 +30,13 @@ int mkdirat_label(int dirfd, const char *path, mode_t mode) {
 }
 
 int mkdir_safe_label(const char *path, mode_t mode, uid_t uid, gid_t gid, MkdirFlags flags) {
-        return mkdir_safe_internal(path, mode, uid, gid, flags, mkdir_label);
+        return mkdir_safe_internal(path, mode, uid, gid, flags, mkdirat_label);
 }
 
 int mkdir_parents_label(const char *path, mode_t mode) {
-        return mkdir_parents_internal(NULL, path, mode, UID_INVALID, UID_INVALID, 0, mkdir_label);
+        return mkdir_parents_internal(NULL, path, mode, UID_INVALID, UID_INVALID, 0, mkdirat_label);
 }
 
 int mkdir_p_label(const char *path, mode_t mode) {
-        return mkdir_p_internal(NULL, path, mode, UID_INVALID, UID_INVALID, 0, mkdir_label);
+        return mkdir_p_internal(NULL, path, mode, UID_INVALID, UID_INVALID, 0, mkdirat_label);
 }