Allow for better interoperability with other mechanisms that would like to update PROMPT_COMMAND
    
    The /etc/profile.d/vte.sh says

    

        # Newer bash versions support PROMPT_COMMAND as an array. In this case

        # only add the __vte_osc7 function to it, and leave setting the terminal

        # title to the outside setup.

        # On older bash, we can only overwrite the whole PROMPT_COMMAND, so must

        # use the __vte_prompt_command function which also sets the title.

    

    Play nice with vte and other tools that might want to just add new stuff to PROMPT_COMMAND.