(ignored-local-variables): Add `dir-local-variables-alist'.

(ignored-local-variables): Add `dir-local-variables-alist'.
(dir-local-variables-alist): New buffer-local variable.
(hack-local-variables-filter): If variable is not dir-local,
i.e. `dir-name' is nil, then remove it from `dir-local-variables-alist',
because file-local overrides dir-local.
(c-postprocess-file-styles)<declare-function>:
Remove obsolete declaration.
(hack-dir-local-variables): Add dir-local variable/value pair to
`dir-local-variables-alist' and remove duplicates.  Doc fix.