E3-1411: Add support for EPICS_DB_INCLUDE_PATH
The e3 environment use the macro EPICS_DB_INCLUDE_PATH to search for template files. This is patch adds support for this for dbLoadTemplate.
One important point is that this patch removes support to the third dbLoadTemplate argument. I could not find it been used anywhere on e3 wrappers. But it's an API break, maybe we need to add it back.