clean up check selection code

This new approach allows filtering out checks from the default set that are
not appropriate for a command, rather than having to list every check
that is appropriate. It also reduces some boilerplate.

Haskell does not define Eq for functions, so I had to go a long way around
with each check having a unique id. Meh.

Command/DropKey.hs

@@ -13,8 +13,8 @@ import qualified Annex
 import Logs.Location
 import Annex.Content
 
-command :: [Command]
-command = [Command "dropkey" (paramRepeating paramKey) defaultChecks seek
+def :: [Command]
+def = [command "dropkey" (paramRepeating paramKey) seek
 	"drops annexed content for specified keys"] 
 
 seek :: [CommandSeek]