ENH: add directive '#sinclude' (or '#includeIfPresent' as long name)

- consistency with make and more succinct.

- reduce code duplication in findEtcFiles.

applications/test/dictionary/testDict

@@ -14,8 +14,8 @@ FoamFile
 }
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
-#includeIfPresent "someUnknownFile"
-#includeIfPresent "$FOAM_CASE/someUnknownFile"
+#sinclude   "someUnknownFile"
+#sinclude   "$FOAM_CASE/someUnknownFile"
 #includeIfPresent "$FOAM_CASE/someUnknownFile-$FOAM_CASENAME"
 
 internalField   uniform 1;