add GETINFO to external protocol (for ronnypfa)

External special remotes can now add info to `git annex info $remote`, by
replying to the GETINFO message.

Had to generalize some helpers to allow consuming multiple messages from
the remote.

The code added to Remote/* here is AGPL licensed, thus changed the license
of the files.

This commit was sponsored by Jake Vosloo on Patreon.

doc/special_remotes/external/example.sh

@@ -285,6 +285,15 @@ while read line; do
 			fi
 		;;
 
+		# This is optional, only provided as an example.
+		GETINFO)
+			echo INFOFIELD "repository location"
+			echo INFOVALUE "$mydirectory"
+			echo INFOFIELD "login"
+			echo INFOVALUE "$MYLOGIN"
+			echo INFOEND
+		;;
+
 		*)
 			echo UNSUPPORTED-REQUEST
 		;;