Only print the path warnings once

We could reset and rerun it on server rebuilds, but that report needs a full build to make sense.

Also clean up the config vs flags in this area: Make all config settings match the flags e.g. `printPathWarnings`, but set up aliases for the
old.

Fixes #11187

hugolib/hugo_sites.go

@@ -77,6 +77,8 @@ type HugoSites struct {
 	contentInit sync.Once
 	content     *pageMaps
 
+	postRenderInit sync.Once
+
 	// Keeps track of bundle directories and symlinks to enable partial rebuilding.
 	ContentChanges *contentChangeMap