install a git-annex-shell shim script when the standalone OSX app runs

I put it in ~/.ssh/ because there's no reliable way to get it into PATH,
and OSX ssh doesn't even honor user's PATH by default.

authorized_keys generators will need to check if it's there. Not done yet.

ui-macos/git-annex.app/Contents/MacOS/git-annex-webapp

@@ -12,8 +12,8 @@ fi
 # If this is a standalone app, set a variable that git-annex can use to
 # install itself.
 if [ -e "$base/bin/git-annex" ]; then
-	GIT_ANNEX_OSX_WEBAPP_PROGRAM="$base/bin/git-annex"
-	export OSX_GIT_ANNEX_APP_PROGRAM
+	GIT_ANNEX_OSX_APP_BASE="$base"
+	export GIT_ANNEX_OSX_APP_BASE
 fi
 
 "$base/runshell" git-annex webapp "$@"

ui-macos/git-annex.app/Contents/MacOS/runshell

@@ -47,7 +47,7 @@ export GIT_EXEC_PATH
 if [ "$1" ]; then
 	cmd="$1"
 	shift 1
-	"$cmd" "$@"
+	exec "$cmd" "$@"
 else
 	$SHELL
 fi