external backends wip

It's able to start them up, the only thing not implemented is generating
and verifying keys. And, the key translation for HasExt.

doc/design/external_backend_protocol.mdwn

@@ -179,11 +179,8 @@ It's important that two different programs don't use the same name, because
 that would result in bad behavior if the wrong program were used with a
 repository with keys generated by the other program.
 
-Here is a list of programs, to avoid picking the same name. Edit this page
-to add yours to the list.
-
-* [[git-annex-backend-XFOO]] is a demo program implementing this protocol
-  with a shell script.
+To avoid picking the same name, there is a list of known external backend
+programs in [[backends]].
 
 ## signals
 

doc/design/external_backend_protocol/git-annex-backend-XFOO

@@ -7,7 +7,7 @@
 
 set -e
 
-hashfile {
+hashfile () {
 	local contentfile="$1"
 	# could send PROGRESS while doing this, but it's
 	# hard to implement that in shell