Fix unix mtime triggers for ud-generate

Previously we only checked if a file had been modified since our last
run.  That didn't catch changes that were only made visible for the next
run.  I.e. a file was changed on some host, ud-generate runs finds no
need to re-generate and stores timestamp X to its statefile.  We rsync
that file to us and on the next ud-geneate run it finds no files
modified since X.