AMIGAOS4: Make isReadable return false for non-existent files

This also properly initializes _bIsValid for non-existent files.
For consistency, isWritable() is changed analogously to isReadable(),
even though it should not lead to changes in behaviour.
AMIGAOS4: Allow getParent() to work for non-directories
AMIGAOS4: Clarify virtual fs root node