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/Helper/AWS.hs

@@ -12,6 +12,7 @@ module Remote.Helper.AWS where
 
 import Annex.Common
 import Creds
+import Types.ProposedAccepted
 
 import qualified Data.Map as M
 import qualified Data.ByteString as B
@@ -23,7 +24,7 @@ creds :: UUID -> CredPairStorage
 creds u = CredPairStorage
 	{ credPairFile = fromUUID u
 	, credPairEnvironment = ("AWS_ACCESS_KEY_ID", "AWS_SECRET_ACCESS_KEY")
-	, credPairRemoteField = "s3creds"
+	, credPairRemoteField = Accepted "s3creds"
 	}
 
 data Service = S3 | Glacier