All command line stuff today..

Added --want-get and --want-drop, which can be used to test preferred content settings
of a repository. For example `git annex find --in . --want-drop` will list the same
files that `git annex drop --auto` would try to drop. (Also renamed `git annex content`
to `git annex wanted`.)

Finally laid to rest problems with `git annex unannex` when multiple files point to the
same key. It's a lot slower, but I'll stop getting bug reports about that.