xdg-wrappers: add mbsync, make wrappers use next-in-path

The new next-in-path utility function finds the next entry for the
wrapped name in PATH after the called script, instead of hardcoding the
usual location for a system install.

lib/next-in-path.zsh

@@ -0,0 +1,12 @@
+# find next entry in PATH for wrapper scripts
+# Usage: next-in-path <program name> <current $0>
+next-in-path() {
+	entries=( $(which -a $1) )
+	index=${entries[(ie)$2]} 
+	if [[ $index -le ${#entries} ]]; then
+		# found $0, use next entry
+		echo $entries[$((index + 1))]
+	else
+		echo $entries[1]
+	fi
+}