remove Params constructor from Utility.SafeCommand

This removes a bit of complexity, and should make things faster
(avoids tokenizing Params string), and probably involve less garbage
collection.

In a few places, it was useful to use Params to avoid needing a list,
but that is easily avoided.

Problems noticed while doing this conversion:

	* Some uses of Params "oneword" which was entirely unnecessary
	  overhead.
	* A few places that built up a list of parameters with ++
	  and then used Params to split it!

Test suite passes.

Remote/Helper/Ssh.hs

@@ -95,7 +95,7 @@ inAnnex r k = do
 {- Removes a key from a remote. -}
 dropKey :: Git.Repo -> Key -> Annex Bool
 dropKey r key = onRemote r (boolSystem, return False) "dropkey"
-	[ Params "--quiet --force"
+	[ Param "--quiet", Param "--force"
 	, Param $ key2file key
 	]
 	[]