Re: about EDITOR_LINENUMBER_SWITCH

Here's a patch to fix what has been discussed:

      * Change EDITOR_LINENUMBER_SWITCH to environment variable.
      * I also changed "switch" to "arg" because "switch" is a bit of a
        sloppy term.
      * So the environment variable is called
        PSQL_EDITOR_LINENUMBER_ARG.
      * Set "+" as hardcoded default value on Unix (since "vi" is the
        hardcoded default editor), so many users won't have to configure
        this at all.
      * I moved the documentation around a bit to centralize the editor
        configuration under environment variables, rather than repeating
        bits of it under every backslash command that invoked an editor.