added GETWANTED, SETWANTED for Tobias's flickr remote

This was unexpectedly difficult because of a depdenency cycle. To parse a
preferred content expression involves several things that need to operate
on the list of remotes. Which needs Remote.External. The only way to avoid
this cycle (I tried breaking it at several points) was to skip parsing the
expression in SETWANTED.

That's sorta ok, because git-annex already has to deal with unparsable
preferred content expressions being stored, in order to handle eg,
upgrades. But I'm still not very happy that I cannot check it.

I feel this is a strong indication that I need to beware of further
bloating the special remote protocol interface.

doc/design/external_special_remote_protocol.mdwn

@@ -212,6 +212,16 @@ in control.
 * `GETUUID`  
   Queries for the UUID of the special remote being used.  
   (git-annex replies with VALUE followed by the UUID.)
+* `SETWANTED PreferredContentExpression`  
+  Can be used to set the preferred content of a repository. Normally
+  this is not configured by a special remote, but it may make sense
+  in some situations to hint at the kind of content that should be stored
+  in the special remote. Note that if a unparsable expression is set,
+  git-annex will ignore it.
+* `GETWANTED`  
+  Gets the current preferred content setting of the repository.
+  (git-annex replies with VALUE followed by the preferred content
+  expression.)
 
 ## general messages