ls: measure the rendered directory header for dired offsets

[ChihweiLHBird]

Summary

--dired offsets must delimit the exact bytes written. write_dir_header measured directory headers by their raw path length, while show_dir_name prints them quoted according to the quoting style in force, so any quoting that lengthens a header (-Q, --quoting-style=shell-escape, a name with a space) cut its //SUBDIRED// entry short and shifted every later //DIRED// offset. show_dir_name now returns the rendered byte length and the bookkeeping uses that. Split out of #14590.

Example

mkdir 'a b' && touch 'a b/x'
ls --dired -R --quoting-style=shell-escape 'a b'

Before: the header is printed as 'a b' (5 bytes) but counted as a b (3 bytes), so //SUBDIRED// 2 5 covers 'a and //DIRED// 62 63 lands on the 0 of the size column instead of x.

  'a b':
  total 0
  -rw-r--r--+ 1 zhiwei zhiwei 0 Sep 17 19:20 x
//DIRED// 62 63
//SUBDIRED// 2 5
//DIRED-OPTIONS// --quoting-style=shell-escape-show-control

After: //SUBDIRED// 2 7 covers 'a b' and //DIRED// 64 65 covers x, the same offsets GNU prints.

  'a b':
  total 0
  -rw-r--r--+ 1 zhiwei zhiwei 0 Sep 17 19:20 x
//DIRED// 64 65
//SUBDIRED// 2 7
//DIRED-OPTIONS// --quoting-style=shell-escape-show-control

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)