Refactor VC merging to fix a layer violation.

* vc/vc.el, vc/vc-cvs.el, vc/vc-rcs.el, vc/vc-svn.el: The 'merge'
backend method of RCS/CVS/SVN is now 'merge-file', to contrast with
'merge-branch'. Prompting for merge revisions is pushed down to the
back ends; this fixes a layering violation that caused bad behavior
with SVN.
ChangeLog typo fix.
* net/nsm.el (network-security-level): Change the default to `medium'.
Test checkin to attempt to tickle a bug reported on the dev list.