dropunused behavior change: Now refuses to drop the last copy of a file, unless you use the --force.

This was the last place in git-annex that could remove data referred to by
the git history, without being forced.

Like drop, dropunused checks remotes, and honors the global annex.numcopies
setting. (However, .gitattributes settings cannot apply to unused files.)

doc/direct_mode.mdwn

@@ -35,7 +35,7 @@ To stop using direct mode:
 
 With direct mode, you're operating without large swathes of git-annex's
 carefully constructed safety net, which ensures that past versions of
-files are preserved and can be accessed (until you dropunused them). 
+files are preserved and can be accessed. 
 With direct mode, any file can be edited directly, or deleted at any time,
 and there's no guarantee that the old version is backed up somewhere else.
 

doc/walkthrough/unused_data.mdwn

@@ -19,7 +19,7 @@ preserving it. So from time to time, you may want to check for such data:
 
 After running `git annex unused`, you can follow the instructions to examine
 the history of files that used the data, and if you decide you don't need that
-data anymore, you can easily remove it:
+data anymore, you can easily remove it from your local repository.
 
 	# git annex dropunused 1
 	dropunused 1 ok