Rename custom_constant_inc to custom_constants_inc

Follow-up to 9531f78d316dd0d3ab628f57ee0faa7e56c5f995

Finishes renaming of custom_constant_inc.php to custom_constantsinc.php
so the name is more consistent with the other custom*_inc.php files.

For v1.2 both files are acceptable with a notice in place that
custom_constant_inc.php will be deprecated in v1.3.