Fix #4537

It should be different from --header-lines-border=none according to the
man page. It should merge two headers unlike the latter.

Options:
  --footer=STR             String to print as footer
  --footer-border[=STYLE]  Draw border around the footer section
                           [rounded|sharp|bold|block|thinblock|double|horizontal|vertical|
                            top|bottom|left|right|line|none] (default: line)
  --footer-label=LABEL     Label to print on the footer border
  --footer-label-pos=COL   Position of the footer label
                           [POSITIVE_INTEGER: columns from left|
                            NEGATIVE_INTEGER: columns from right][:bottom]
                           (default: 0 or center)

The default border type for footer is 'line', which draws a single
separator between the footer and the list. It changes its position
depending on `--layout`, so you don't have to manually switch between
'top' and 'bottom'

The 'line' style is now supported by other border types as well.
`--list-border` is the only exception.

fzf displayed --header-lines inconsistently depending on the presence of borders:

  # --header and --header-lines co-located
  seq 10 | fzf --header-lines 3 --header "$(seq 101 103)" --header-first

  # --header and --header-lines separated
  seq 10 | fzf --header-lines 3 --header "$(seq 101 103)" --header-first --header-lines-border

This commit fixes the inconsistency with the following logic:

* If only one of --header or --header-lines is provided, --header-first
  applies to that single header.
* If both are present, --header-first affects only the regular --header,
  not --header-lines.

Close #4390

Fix #4227