Fix #10518, PAL engine settings no longer honored by qgis_mapserv

This attempts to fix the issue. Works fine here.

Do the `drawOverlays` and `loadLabelSettings` functions need to be virtual in the base `QgsWMSConfigParser` class? Or, should they only extend in `QgsWMSProjectParser`, since a project is required?

The `mDrawOutlineLabels` extra setting comment, with no code, refers to the setting added by PR #1286.