be stricter about rejecting invalid configurations for remotes

This is a first step toward that goal, using the ProposedAccepted type
in RemoteConfig lets initremote/enableremote reject bad parameters that
were passed in a remote's configuration, while avoiding enableremote
rejecting bad parameters that have already been stored in remote.log

This does not eliminate every place where a remote config is parsed and a
default value is used if the parse false. But, I did fix several
things that expected foo=yes/no and so confusingly accepted foo=true but
treated it like foo=no. There are still some fields that are parsed with
yesNo but not not checked when initializing a remote, and there are other
fields that are parsed in other ways and not checked when initializing a
remote.

This also lays groundwork for rejecting unknown/typoed config keys.

Remote/Bup.hs

@@ -33,6 +33,7 @@ import Utility.UserInfo
 import Annex.UUID
 import Annex.Ssh
 import Utility.Metered
+import Types.ProposedAccepted
 
 type BupRepo = String
 
@@ -108,8 +109,8 @@ bupSetup _ mu _ c gc = do
 	u <- maybe (liftIO genUUID) return mu
 
 	-- verify configuration is sane
-	let buprepo = fromMaybe (giveup "Specify buprepo=") $
-		M.lookup "buprepo" c
+	let buprepo = maybe (giveup "Specify buprepo=") fromProposedAccepted $
+		M.lookup (Accepted "buprepo") c
 	(c', _encsetup) <- encryptionSetup c gc
 
 	-- bup init will create the repository.