Some fixes for vc-ignore.

Some fixes for vc-ignore.

* lisp/vc/vc.el (vc-ignore): Rewrite.
(vc-default-ignore): New function.
(vc-default-ignore-completion-table): Use find-ignore-file.

* lisp/vc/vc-bzr.el (vc-bzr-ignore, vc-bzr-ignore-completion-table):
* lisp/vc/vc-git.el (vc-git-ignore, vc-git-ignore-completion-table):
* lisp/vc/vc-hg.el (vc-hg-ignore, vc-hg-ignore-completion-table):
Remove.  Most code moved to vc.el.

* doc/emacs/maintaining.texi (VC Ignore): Mention `vc-ignore' with prefix argument.