Hide default separator when a border separates input from list

- Applies when adjacent section (header, header lines, or list) draws
  border line facing input section

    # No separator below header border
    fzf --style full --input-border none --header foo

    # Separator shown; nothing separates input from list
    fzf --style full --input-border none --header foo --no-header-border

- Evaluated dynamically so change-header/toggle-header update it
- Inline header is drawn inside list border and not adjacent to input;
  decide from section actually next to it
- Separator is kept when a preview window can be at 'next' position,
  right next to the input section

man/man1/fzf.1

@@ -823,6 +823,10 @@ A synonym for \fB\-\-info=hidden\fB
 The given string will be repeated to form the horizontal separator on the info
 line (default: '─' or '\-' depending on \fB\-\-no\-unicode\fR).
 
+Unless explicitly specified, the separator is not displayed if the input
+section is already visually separated from the list section by a border line
+(e.g. \fB\-\-input\-border\fR or \fB\-\-header\-border\fR).
+
 ANSI color codes are supported.
 
 .TP