Make marking conflicted files as resolved upon saving opt-out

Make marking conflicted files as resolved upon saving opt-out

This fixes Bug#3860.

* lisp/vc/vc.el (vc-resolve-conflicts): New user option.
* lisp/vc/vc-bzr.el (vc-bzr-resolve-conflicts): New user option.
(vc-bzr-find-file-hook): Use it.
* lisp/vc/vc-hg.el (vc-hg-resolve-conflicts): New user option.
(vc-hg-find-file-hook): Use it.
* lisp/vc/vc-svn.el (vc-svn-resolve-conflicts): New user option.
(vc-svn-find-file-hook): Use it.
* lisp/vc/vc-git.el (vc-git-resolve-conflicts): Support
'default' as an option.  Adjust docstring and version.
(vc-git-find-file-hook): Respect vc-resolve-conflicts.
* etc/NEWS: Announce the new options.

(cherry picked from commit acb96a5ca8ac3bef80ca2ff1496cacb3ab57c87a)