cmd: Use private custom func to fix clash between rclone and kubectl

Before this change, rclone used the `__custom_func` hook to control
the completions of remote files.  However this clashes with other
cobra users, the most notable example being kubectl.

Upgrading cobra to master allows us to use a namespaced function
`__rclone_custom_func` which fixes the problem.

Fixes #1529

cmd/help.go

@@ -37,7 +37,7 @@ documentation, changelog and configuration walkthroughs.
 
 const (
 	bashCompletionFunc = `
-__custom_func() {
+__rclone_custom_func() {
     if [[ ${#COMPREPLY[@]} -eq 0 ]]; then
         local cur cword prev words
         if declare -F _init_completion > /dev/null; then